Method and apparatus for analysing the condition of a machine having a rotating part

ABSTRACT

A method analyzing a machine having a rotating shaft includes generating an electric measurement signal dependent on mechanical vibrations from the shaft rotation; sampling the measurement signal to generate a digital measurement data signal; performing a decimation of the digital measurement data signal to achieve a digital signal having a reduced sampling frequency, where the decimation includes controlling the reduced sampling frequency such that the number of sample values per revolution of the shaft is kept at a substantially constant value, and receiving the digital signal at an enhancer input performing a correlation in the enhancer so as to produce an output signal sequence where repetitive signals amplitude components are amplified in relation to stochastic signal components, and performing a condition analysis for analyzing the condition of the machine dependent on the digital signal having a reduced sampling frequency.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method for analysing the condition ofa machine, and to an apparatus for analysing the condition of a machine.The invention also relates to a system including such an apparatus andto a method of operating such an apparatus. The invention also relatesto a computer program for causing a computer to perform an analysisfunction.

DESCRIPTION OF RELATED ART

Machines with moving parts are subject to wear with the passage of time,which often causes the condition of the machine to deteriorate. Examplesof such machines with movable parts are motors, pumps, generators,compressors, lathes and CNC-machines. The movable parts may comprise ashaft and bearings.

In order to prevent machine failure, such machines should be subject tomaintenance, depending on the condition of the machine. Therefore theoperating condition of such a machine is preferably evaluated from timeto time. The operating condition can be determined by measuringvibrations emanating from a bearing or by measuring temperature on thecasing of the machine, which temperatures are dependent on the operatingcondition of the bearing. Such condition checks of machines withrotating or other moving parts are of great significance for safety andalso for the length of the life of such machines. It is known tomanually perform such measurements on machines. This ordinarily is doneby an operator with the help of a measuring instrument performingmeasurements at measuring points on one or several machines.

A number of commercial instruments are available, which rely on the factthat defects in rolling-element bearings generate short pulses, usuallycalled shock pulses. A shock pulse measuring apparatus may generateinformation indicative of the condition of a bearing or a machine.

WO 03062766 discloses a machine having a measuring point and a shaftwith a certain shaft diameter, wherein the shaft can rotate when themachine is in use. WO 03062766 also discloses an apparatus for analysingthe condition of a machine having a rotating shaft. The disclosedapparatus has a sensor for producing a measured value indicatingvibration at a measuring point. The apparatus disclosed in WO 03062766has a data processor and a memory. The memory may store program codewhich, when run on the data processor, will cause the analysis apparatusto perform a Machine Condition Monitoring function. Such a MachineCondition Monitoring function may include shock pulse measuring.

U.S. Pat. No. 6,053,047 discloses an accelerometer used as vibrationsensor collecting analog vibration data which is delivered to anA/D-converter which provides digital vibration data to a processor 90.According to U.S. Pat. No. 6,053,047 the processor performs digitalbandpass filtering of digital vibration data, rectifying the filteredsignal, and low pass filtering the rectified signal to produce a lowfrequency signal. The low frequency signal is passed through a capacitorto produce a demodulated signal. An FFT is performed on the demodulatedsignal 116 to produce a vibration spectrum. U.S. Pat. No. 6,053,047 alsoteaches to calculate the resonant frequency of each physical path fromthe accelerometer to various vibration sources in the motor and U.S.Pat. No. 6,053,047 teaches to perform this calibration step before themotor leaves the factory. Alternatively such calibration of eachphysical path from the various vibration sources to the accelerometermust be performed using a calibrated hammer, according to U.S. Pat. No.6,053,047.

SUMMARY

An aspect of the invention relates to an apparatus for analysing thecondition of a machine having a part rotating with a speed of rotation,comprising:

-   -   a first sensor adapted to generate an analogue electric        measurement signal (S_(EA)) dependent on mechanical vibrations        emanating from rotation of said part;        an analogue-to-digital converter (44) for sampling said analogue        measurement signal at a sampling frequency (f_(S)) so as to        generate a digital measurement data signal (S_(MD)) in response        to said received analogue measurement data; said digital        measurement data signal (S_(MD)) having a first        Signal-to-Noise-Ratio level;    -   a first decimator for performing a decimation of the digital        measurement data signal (S_(MD), S_(ENV)) so as to achieve a        first digital signal (S_(MD), S_(ENV)) having a first reduced        sampling frequency (f_(SR1));    -   a second decimator (470, 470A, 470B), said second decimator        (470, 470A, 470B) having        -   a first input for receiving said first digital signal            (S_(MD), S_(ENV)) and        -   a second input for receiving a signal indicative of a            variable speed of rotation (f_(ROT)) associated with said            part;        -   a third input for receiving a signal indicative of an output            sample rate setting signal;        -   said second decimator (470, 470A, 470B) being adapted to            generate a second digital signal (S_(RED2)) having a second            reduced sampling frequency (f_(SR2)) in response to        -   said first digital signal (S_(MD), S_(ENV)),        -   said signal indicative of a relevant speed of rotation            (f_(ROT)) and        -   said signal indicative of an output sample rate setting            signal such that the number of sample values per revolution            of said rotating part is kept at a substantially constant            value; and    -   an enhancer having an input for receiving said second digital        signal (S_(RED2)); said enhancer being adapted to receive a        first plurality (I_(LENGTH)) of sample values, wherein said        second digital signal (S_(RED2)) represents mechanical        vibrations emanating from rotation of said part for a duration        of time;    -   said enhancer being adapted to perform a correlation so as to        produce an output signal sequence (O) wherein repetitive signals        amplitude components are amplified in relation to stochastic        signal components.    -   an evaluator (230) for performing a condition analysis function        (F1, F2, Fn) for analysing the condition of the machine        dependent on said second digital signal (S_(RED2)).

According to an embodiment of the apparatus said first decimator isadapted to reduce the sampling rate by an integer factor (M).

An aspect B1 of the invention relates to a computer program for causinga computer to analyse the condition of a machine having a slowlyrotating part, the computer program comprising:

-   -   computer readable code means which, when run on a computer,        causes the computer to generate an analogue electric measurement        signal (S_(EA)) dependent on mechanical vibrations emanating        from rotation of said shaft;    -   computer readable code means which, when run on a computer,        causes the computer to sample said analogue measurement signal        at a sampling frequency (f_(S)) so as to generate a digital        measurement data signal (S_(MD)) in response to said received        analogue measurement data;    -   computer readable code means which, when run on a computer,        causes the computer to perform a decimation of the digital        measurement data signal (S_(MD)) so as to achieve a digital        signal (S_(RED)) having a reduced sampling frequency (f_(SR1),        f_(SR2));    -   computer readable code means which, when run on a computer,        causes the computer to control the reduced sampling frequency        (f_(SR1), f_(SR2)) such that the number of sample values per        revolution of the shaft (8) is kept at a substantially constant        value; and    -   computer readable code means which, when run on a computer,        causes the computer to perform a condition analysis function        (F1, F2, Fn) for analysing the condition of the machine        dependent on said digital signal (S_(RED)) having a reduced        sampling frequency (f_(SR1), f_(SR2)).

A computer program product comprising

-   -   a computer readable medium; and        a computer program according to claim aspect B1,        said computer program being recorded on said computer readable        medium.

The invention also relates to a condition monitoring system comprising

-   -   A shock pulse measuring sensor associated with an epicyclic gear        system (700) for generating an analogue signal;    -   An A/D converter coupled to receive said analogue signal;    -   A plurality of signal processing functions (94, 240, 250, 310,        470, 320)

The invention also relates to a method of operating a finite impulseresponse filter having an input (480) for receiving detected input datavalues (S(j)) of a digital measurement data signal (S_(MD)) dependent onmechanical vibrations emanating from rotation of a shaft, said digitalmeasurement data signal (S_(MD)) having a sampling frequency (f_(SR1));and an input for receiving a signal indicative of a speed of rotation ofa monitored rotating part at a time associated said detection of saidinput data values (S(j)); and a memory (604) adapted to receive andstore said data values (S(j)) and information indicative of thecorresponding speed of rotation (f_(ROT)); and a value generator (606)adapted to generate a fractional value (D); and; a plurality of FIRfilter taps having individual filter values; the method comprising thestep of interpolating a filter value.

BRIEF DESCRIPTION OF THE DRAWINGS

For simple understanding of the present invention, it will be describedby means of examples and with reference to the accompanying drawings, ofwhich:

FIG. 1 shows a schematic block diagram of an embodiment of a conditionanalyzing system 2 according to an embodiment of the invention.

FIG. 2A is a schematic block diagram of an embodiment of a part of thecondition analyzing system 2 shown in FIG. 1.

FIG. 2B is a schematic block diagram of an embodiment of a sensorinterface.

FIG. 2C is an illustration of a measuring signal from a vibrationsensor.

FIG. 2D illustrates a measuring signal amplitude generated by a shockpulse sensor.

FIG. 2E illustrates a measuring signal amplitude generated by avibration sensor.

FIG. 3 is a simplified illustration of a Shock Pulse Measurement sensoraccording to an embodiment of the invention.

FIG. 4 is a simplified illustration of an embodiment of the memory 60and its contents.

FIG. 5 is a schematic block diagram of an embodiment of the analysisapparatus at a client location with a machine 6 having a movable shaft.

FIG. 6 illustrates a schematic block diagram of an embodiment of thepre-processor according to an embodiment of the present invention.

FIG. 7 illustrates an embodiment of the evaluator 230.

FIG. 8 illustrates another embodiment of the evaluator 230.

FIG. 9 illustrates another embodiment of the pre-processor 200.

FIG. 10A is a flow chart that illustrates embodiments of a method forenhancing repetitive signal patterns in signals.

FIG. 10B is a flow chart illustrating a method of generating a digitaloutput signal.

FIG. 11 is a schematic illustration of a first memory having pluralmemory positions

FIG. 12 is a schematic illustration of a second memory having pluralmemory positions t.

FIG. 13 is a schematic illustration of an example output signal S_(MDP)comprising two repetitive signals signatures.

FIG. 14A illustrates a number of sample values in the signal deliveredto the input of the decimator 310.

FIG. 14B illustrates output sample values of the corresponding timeperiod.

FIG. 15A illustrates a decimator according to an embodiment of theinvention.

FIG. 15B illustrates another embodiment of the invention

FIG. 16 illustrates an embodiment of the invention including a decimatorand an enhancer, as described above, and a fractional decimator.

FIG. 17 illustrates an embodiment of the fractional decimator.

FIG. 18 illustrates another embodiment of the fractional decimator.

FIG. 19 illustrates decimator and another embodiment of fractionaldecimator.

FIG. 20 is a block diagram of decimator and yet another embodiment offractional decimator.

FIG. 21 is a flow chart illustrating an embodiment of a method ofoperating the decimator and the fractional decimator of FIG. 20.

FIGS. 22A, 22B & 22C describe a method which may be implemented as acomputer program.

FIG. 23 is a front view illustrating an epicyclic gear system

FIG. 24 is a schematic side view of the epicyclic gear system 700 ofFIG. 23, as seen in the direction of the arrow SW in FIG. 23.

FIG. 25 illustrates an analogue version of an exemplary signal producedby and outputted by the pre-processor 200 (see FIG. 5 or FIG. 16) inresponse to signals detected by the at least one sensor 10 upon rotationof the epicyclic gear system.

FIG. 26 illustrates an example of a portion of the high amplitude region702A of the signal shown in FIG. 25.

FIG. 27 illustrates an exemplary frequency spectrum of a signalcomprising a small periodic disturbance 903 as illustrated in FIG. 26.

FIG. 28 illustrates an example of a portion of the exemplary signalshown in FIG. 25.

FIG. 29 illustrates yet an embodiment of a condition analyzing systemaccording to an embodiment of the invention.

FIG. 30 is a block diagram illustrating the parts of the signalprocessing arrangement of FIG. 29 together with the user interface andthe display.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following description similar features in different embodimentsmay be indicated by the same reference numerals.

FIG. 1 shows a schematic block diagram of an embodiment of a conditionanalyzing system 2 according to an embodiment of the invention.Reference numeral 4 relates to a client location with a machine 6 havinga movable part 8. The movable part may comprise bearings 7 and a shaft 8which, when the machine is in operation, rotates. The operatingcondition of the shaft 8 or of a bearing 7 can be determined in responseto vibrations emanating from the shaft and/or bearing when the shaftrotates. The client location 4, which may also be referred to as clientpart or user part, may for example be the premises of a wind farm, i.e.a group of wind turbines at a location, or the premises of a paper millplant, or some other manufacturing plant having machines with movableparts.

An embodiment of the condition analyzing system 2 is operative when asensor 10 is attached on or at a measuring point 12 on the body of themachine 6. Although FIG. 1 only illustrates two measuring points 12, itto be understood that a location 4 may comprise any number of measuringpoints 12. The condition analysis system 2 shown in FIG. 1, comprises ananalysis apparatus 14 for analysing the condition of a machine on thebasis of measurement values delivered by the sensor 10.

The analysis apparatus 14 has a communication port 16 for bi-directionaldata exchange. The communication port 16 is connectable to acommunications network 18, e.g. via a data interface 19. Thecommunications network 18 may be the world wide internet, also known asthe Internet. The communications network 18 may also comprise a publicswitched telephone network.

A server computer 20 is connected to the communications network 18. Theserver 20 may comprise a database 22, user input/output interfaces 24and data processing hardware 26, and a communications port 29. Theserver computer 20 is located on a location 28, which is geographicallyseparate from the client location 4. The server location 28 may be in afirst city, such as the Swedish capital Stockholm, and the clientlocation may be in another city, such as Stuttgart, Germany or Detroitin Michigan, USA. Alternatively, the server location 28 may be in afirst part of a town and the client location may be in another part ofthe same town. The server location 28 may also be referred to assupplier part 28, or supplier part location 28.

According to an embodiment of the invention a central control location31 comprises a control computer 33 having data processing hardware andsoftware for surveying a plurality of machines at the client location 4.The machines 6 may be wind turbines or gear boxes used in wind turbines.Alternatively the machines may include machinery in e.g. a paper mill.The control computer 33 may comprise a database 22B, user input/outputinterfaces 24B and data processing hardware 26B, and a communicationsport 29B. The central control location 31 may be separated from theclient location 4 by a geographic distance. By means of communicationsport 29B the control computer 33 can be coupled to communicate withanalysis apparatus 14 via port 16. The analysis apparatus 14 may delivermeasurement data being partly processed so as to allow further signalprocessing and/or analysis to be performed at the central location 31 bycontrol computer 33.

A supplier company occupies the supplier part location 28. The suppliercompany may sell and deliver analysis apparatuses 14 and/or software foruse in an analysis apparatus 14. The supplier company may also sell anddeliver analysis software for use in the control computer at the centralcontrol location 31. Such analysis software 94,105 is discussed inconnection with FIG. 4 below. Such analysis software 94,105 may bedelivered by transmission over said communications network 18.

According to one embodiment of the system 2 the apparatus 14 is aportable apparatus which may be connected to the communications network18 from time to time.

According to another embodiment of the system 2 the apparatus 14 isconnected to the communications network 18 substantially continuously.Hence, the apparatus 14 according to this embodiment may substantiallyalways be “on line” available for communication with the suppliercomputer 20 and/or with the control computer 33 at control location 31.

FIG. 2A is a schematic block diagram of an embodiment of a part of thecondition analyzing system 2 shown in FIG. 1. The condition analyzingsystem, as illustrated in FIG. 2A, comprises a sensor unit 10 forproducing a measured value. The measured value may be dependent onmovement or, more precisely, dependent on vibrations or shock pulsescaused by bearings when the shaft rotates.

An embodiment of the condition analyzing system 2 is operative when adevice 30 is firmly mounted on or at a measuring point on a machine 6.The device 30 mounted at the measuring point may be referred to as astud 30. A stud 30 can comprise a connection coupling 32 to which thesensor unit 10 is removably attachable. The connection coupling 32 can,for example comprise double start threads for enabling the sensor unitto be mechanically engaged with the stud by means of a ¼ turn rotation.

A measuring point 12 can comprise a threaded recess in the casing of themachine. A stud 30 may have a protruding part with threads correspondingto those of the recess for enabling the stud to be firmly attached tothe measuring point by introduction into the recess like a bolt.

Alternatively, a measuring point can comprise a threaded recess in thecasing of the machine, and the sensor unit 10 may comprise correspondingthreads so that it can be directly introduced into the recess.Alternatively, the measuring point is marked on the casing of themachine only with a painted mark.

The machine 6 exemplified in FIG. 2A may have a rotating shaft with acertain shaft diameter d1. The shaft in the machine 24 may rotate with aspeed of rotation V1 when the machine 6 is in use.

The sensor unit 10 may be coupled to the apparatus 14 for analysing thecondition of a machine. With reference to FIG. 2A, the analysisapparatus 14 comprises a sensor interface 40 for receiving a measuredsignal or measurement data, produced by the sensor 10. The sensorinterface 40 is coupled to a data processing means 50 capable ofcontrolling the operation of the analysis apparatus 14 in accordancewith program code. The data processing means 50 is also coupled to amemory 60 for storing said program code.

According to an embodiment of the invention the sensor interface 40comprises an input 42 for receiving an analogue signal, the input 42being connected to an analogue-to-digital (A/D) converter 44, thedigital output 48 of which is coupled to the data processing means 50.The A/D converter 44 samples the received analogue signal with a certainsampling frequency f_(S) so as to deliver a digital measurement datasignal S_(MD) having said certain sampling frequency f_(S) and whereinthe amplitude of each sample depends on the amplitude of the receivedanalogue signal at the moment of sampling.

According to another embodiment of the invention, illustrated in FIG.2B, the sensor interface 40 comprises an input 42 for receiving ananalogue signal S_(EA) from a Shock Pulse Measurement Sensor, aconditioning circuit 43 coupled to receive the analogue signal, and anA/D converter 44 coupled to receive the conditioned analogue signal fromthe conditioning circuit 43. The A/D converter 44 samples the receivedconditioned analogue signal with a certain sampling frequency f_(S) soas to deliver a digital measurement data signal S_(MD) having saidcertain sampling frequency f_(S) and wherein the amplitude of eachsample depends on the amplitude of the received analogue signal at themoment of sampling.

The sampling theorem guarantees that bandlimited signals (i.e., signalswhich have a maximum frequency) can be reconstructed perfectly fromtheir sampled version, if the sampling rate f_(S) is more than twice themaximum frequency f_(SEAmax) of the analogue signal S_(EA) to bemonitored. The frequency equal to one-half of the sampling rate istherefore a theoretical limit on the highest frequency that can beunambiguously represented by the sampled signal S_(MD). This frequency(half the sampling rate) is called the Nyquist frequency of the samplingsystem. Frequencies above the Nyquist frequency f_(N) can be observed inthe sampled signal, but their frequency is ambiguous. That is, afrequency component with frequency f cannot be distinguished from othercomponents with frequenciesB*f _(N) +f,andB*f _(N) −ffor nonzero integers B. This ambiguity, known as aliasing may be handledby filtering the signal with an anti-aliasing filter (usually a low-passfilter with cutoff near the Nyquist frequency) before conversion to thesampled discrete representation.

In order to provide a safety margin for in terms of allowing a non-idealfilter to have a certain slope in the frequency response, the samplingfrequency may be selected to a higher value than 2. Hence, according toembodiments of the invention the sampling frequency may be set tof _(S) =k*f _(SEAmax)wherein

-   -   k is a factor having a value higher than 2.0

Accordingly the factor k may be selected to a value higher than 2.0.Preferably factor k may be selected to a value between 2.0 and 2.9 inorder to provide a good safety margin while avoiding to generateunnecessarily many sample values. According to an embodiment the factork is advantageously selected such that 100*k/2 renders an integer.According to an embodiment the factor k may be set to 2.56. Selecting kto 2.56 renders 100*k=256=2 raised to 8.

According to an embodiment the sampling frequency f_(S) of the digitalmeasurement data signal S_(MD) may be fixed to a certain value f_(S),such as e.g. f_(S)=102 kHz

Hence, when the sampling frequency f_(S) is fixed to a certain valuef_(S), the maximum frequency f_(SEAmax) of the analogue signal S_(EA)will be:f _(SEAmax) =f _(S) /kwherein f_(SEAmax) is the highest frequency to be analyzed in thesampled signal

Hence, when the sampling frequency f_(S) is fixed to a certain valuef_(S)=102 400 Hz, and the factor k is set to 2.56, the maximum frequencyf_(SEAmax) of the analogue signal S_(EA) will be:f _(SEAmax) =f _(S) /k=102 400/2.56=40 kHz

Accordingly, a digital measurement data signal S_(MD), having a certainsampling frequency f_(S), is generated in response to said receivedanalogue measurement signal S_(EA). The digital output 48 of the A/Dconverter 44 is coupled to the data processing means 50 via an output 49of the sensor interface 40 so as to deliver the digital measurement datasignal S_(MD) to the data processing means 50.

The sensor unit 10 may comprise a vibration transducer, the sensor unitbeing structured to physically engage the connection coupling of themeasuring point so that vibrations of the machine at the measuring pointare transferred to the vibration transducer. According to an embodimentof the invention the sensor unit comprises a transducer having apiezo-electric element. When the measuring point 12 vibrates, the sensorunit 10, or at least a part of it, also vibrates and the transducer thenproduces an electrical signal of which the frequency and amplitudedepend on the mechanical vibration frequency and the vibration amplitudeof the measuring point 12, respectively. According to an embodiment ofthe invention the sensor unit 10 is a vibration sensor, providing ananalogue amplitude signal of e.g. 10 mV/g in the Frequency Range 1.00 to10000 Hz. Such a vibration sensor is designed to deliver substantiallythe same amplitude of 10 mV irrespective of whether it is exerted to theacceleration of 1 g (9.82 m/s²) at 1 Hz, 3 Hz or 10 Hz. Hence, a typicalvibration sensor has a linear response in a specified frequency range upto around 10 kHz. Mechanical vibrations in that frequency rangeemanating from rotating machine parts are usually caused by imbalance ormisalignment. However, when mounted on a machine the linear responsevibration sensor typically also has several different mechanicalresonance frequencies dependent on the physical path between sensor andvibration source.

A damage in a roller bearing causes relatively sharp elastic waves,known as shock pulses, travelling along a physical path in the housingof a machine before reaching the sensor. Such shock pulses often have abroad frequency spectrum. The amplitude of a roller bearing shock pulseis typically lower than the amplitude of a vibration caused by imbalanceor misalignment.

The broad frequency spectrum of shock pulse signatures enables them toactivate a “ringing response” or a resonance at a resonance frequencyassociated with the sensor. Hence, a typical measuring signal from avibration sensor may have a wave form as shown in FIG. 2C, i.e. adominant low frequency signal with a superimposed higher frequency loweramplitude resonant “ringing response”.

In order to enable analysis of the shock pulse signature, oftenemanating from a bearing damage, the low frequency component must befiltered out. This can be achieved by means of a high pass filter or bymeans of a band pass filter. However, these filters must be adjustedsuch that the low frequency signal portion is blocked while the highfrequency signal portion is passed on. An individual vibration sensorwill typically have one resonance frequency associated with the physicalpath from one shock pulse signal source, and a different resonancefrequency associated with the physical path from another shock pulsesignal source, as mentioned in U.S. Pat. No. 6,053,047. Hence, filteradjustment aiming to pass high the frequency signal portion requiresindividual adaptation when a vibration sensor is used.

When such filter is correctly adjusted the resulting signal will consistof the shock pulse signature(s). However, the analysis of the shockpulse signature(s) emanating from a vibration sensor is somewhatimpaired by the fact that the amplitude response as well as resonancefrequency inherently varies dependent on the individual physical pathfrom the shock pulse signal sources.

Advantageously, these drawbacks associated with vibration sensors may bealleviated by the use of a Shock Pulse Measurement sensor. The ShockPulse Measurement sensor is designed and adapted to provide apre-determined mechanical resonance frequency, as described in furtherdetail below.

This feature of the Shock Pulse Measurement sensor advantageouslyrenders repeatable measurement results in that the output signal from aShock Pulse Measurement sensor has a stable resonance frequencysubstantially independent on the physical path between the irrespectivebetween the shock pulse signal source and the shock pulse sensor.Moreover, mutually different individual shock pulse sensors provide avery small, if any, deviation in resonance frequency.

An advantageous effect of this is that signal processing is simplified,in that filters need not be individually adjusted, in contrast to thecase described above when vibration sensors are used. Moreover, theamplitude response from shock pulse sensors is well defined such that anindividual measurement provides reliable information when measurement isperformed in accordance with appropriate measurement methods defined byS.P.M. Instrument AB.

FIG. 2D illustrates a measuring signal amplitude generated by a shockpulse sensor, and FIG. 2E illustrates a measuring signal amplitudegenerated by a vibration sensor. Both sensors have been exerted to thesame series of mechanical shocks without the typical low frequencysignal content. As clearly seen in FIGS. 2D and 2E, the duration of aresonance response to a shock pulse signature from the Shock PulseMeasurement sensor is shorter than the corresponding resonance responseto a shock pulse signature from the vibration sensor.

This feature of the Shock Pulse Measurement sensor of providing distinctshock pulse signature responses has the advantageous effect of providinga measurement signal from which it is possible to distinguish betweendifferent mechanical shock pulses that occur within a short time span.

According to an embodiment of the invention the sensor is a Shock PulseMeasurement sensor. FIG. 3 is a simplified illustration of a Shock PulseMeasurement sensor 10 according to an embodiment of the invention.According to this embodiment the sensor comprises a part 110 having acertain mass or weight and a piezo-electrical element 120. Thepiezo-electrical element 120 is somewhat flexible so that it cancontract and expand when exerted to external force. The piezo-electricalelement 120 is provided with electrically conducting layers 130 and 140,respectively, on opposing surfaces. As the piezo-electrical element 120contracts and expands it generates an electric signal which is picked upby the conducting layers 130 and 140. Accordingly, a mechanicalvibration is transformed into an analogue electrical measurement signalS_(EA), which is delivered on output terminals 145, 150.

The piezo-electrical element 120 may be positioned between the weight110 and a surface 160 which, during operation, is physically attached tothe measuring point 12, as illustrated in FIG. 3.

The Shock Pulse Measurement sensor 10 has a resonance frequency thatdepends on the mechanical characteristics for the sensor, such as themass m of weight part 110 and the resilience of piezo-electrical element120. Hence, the piezo-electrical element has an elasticity and a springconstant k. The mechanical resonance frequency f_(RM) for the sensor istherefore also dependent on the mass m and the spring constant k.

According to an embodiment of the invention the mechanical resonancefrequency f_(RM) for the sensor can be determined by the equationfollowing equation:f _(RM)=1/(2π)√(k/m)  (eq1)

According to another embodiment the actual mechanical resonancefrequency for a Shock Pulse Measurement sensor 10 may also depend onother factors, such as the nature of the attachment of the sensor 10 tothe body of the machine 6.

The resonant Shock Pulse Measurement sensor 10 is thereby particularlysensitive to vibrations having a frequency on or near the mechanicalresonance frequency f_(RM). The Shock Pulse Measurement sensor 10 may bedesigned so that the mechanical resonance frequency f_(RM) is somewherein the range from 28 kHz to 37 kHz. According to another embodiment themechanical resonance frequency f_(RM) is somewhere in the range from 30kHz to 35 kHz.

Accordingly the analogue electrical measurement signal has an electricalamplitude which may vary over the frequency spectrum. For the purpose ofdescribing the theoretical background, it may be assumed that if theShock Pulse Measurement sensor 10 were exerted to mechanical vibrationswith identical amplitude in all frequencies from e.g. 1 Hz to e.g. 200000 kHz, then the amplitude of the analogue signal S_(EA) from the ShockPulse Measurement Sensor will have a maximum at the mechanical resonancefrequency f_(RM), since the sensor will resonate when being “pushed”with that frequency.

With reference to FIG. 2B, the conditioning circuit 43 receives theanalogue signal S_(EA). The conditioning circuit 43 may be designed tobe an impedance adaption circuit designed to adapt the input impedanceof the A/D-converter as seen from the sensor terminals 145,150 so thatan optimum signal transfer will occur. Hence, the conditioning circuit43 may operate to adapt the input impedance Z_(in) as seen from thesensor terminals 145,150 so that a maximum electric power is deliveredto the A/D-converter 44. According to an embodiment of the conditioningcircuit 43 the analogue signal S_(EA) is fed to the primary winding of atransformer, and a conditioned analogue signal is delivered by asecondary winding of the transformer. The primary winding has n1 turnsand the secondary winding has n2 turns, the ratio n1/n2=n₁₂. Hence, theA/D converter 44 is coupled to receive the conditioned analogue signalfrom the conditioning circuit 43. The A/D converter 44 has an inputimpedance Z₄₄, and the input impedance of the A/D-converter as seen fromthe sensor terminals 145,150 will be (n1/n2)²*Z₄₄, when the conditioningcircuit 43 is coupled in between the sensor terminals 145,150 and theinput terminals of the A/D converter 44.

The A/D converter 44 samples the received conditioned analogue signalwith a certain sampling frequency f_(S) so as to deliver a digitalmeasurement data signal S_(MD) having said certain sampling frequencyf_(S) and wherein the amplitude of each sample depends on the amplitudeof the received analogue signal at the moment of sampling.

According to embodiments of the invention the digital measurement datasignal S_(MD) is delivered to a means 180 for digital signal processing(See FIG. 5).

According to an embodiment of the invention the means 180 for digitalsignal processing comprises the data processor 50 and program code forcausing the data processor 50 to perform digital signal processing.According to an embodiment of the invention the processor 50 is embodiedby a Digital Signal Processor. The Digital Signal Processor may also bereferred to as a DSP.

With reference to FIG. 2A, the data processing means 50 is coupled to amemory 60 for storing said program code. The program memory 60 ispreferably a non-volatile memory. The memory 60 may be a read/writememory, i.e. enabling both reading data from the memory and writing newdata onto the memory 60. According to an embodiment the program memory60 is embodied by a FLASH memory. The program memory 60 may comprise afirst memory segment 70 for storing a first set of program code 80 whichis executable so as to control the analysis apparatus 14 to performbasic operations (FIG. 2A and FIG. 4). The program memory may alsocomprise a second memory segment 90 for storing a second set of programcode 94. The second set of program code 94 in the second memory segment90 may include program code for causing the analysis apparatus toprocess the detected signal, or signals, so as to generate apre-processed signal or a set of pre-processed signals. The memory 60may also include a third memory segment 100 for storing a third set ofprogram code 104. The set of program code 104 in the third memorysegment 100 may include program code for causing the analysis apparatusto perform a selected analysis function 105. When an analysis functionis executed it may cause the analysis apparatus to present acorresponding analysis result on user interface 106 or to deliver theanalysis result on port 16 (See FIG. 1 and FIG. 2A and FIGS. 7 and 8).

The data processing means 50 is also coupled to a read/write memory 52for data storage. Moreover, the data processing means 50 may be coupledto an analysis apparatus communications interface 54. The analysisapparatus communications interface 54 provides for bi-directionalcommunication with a measuring point communication interface 56 which isattachable on, at or in the vicinity of the measuring point on themachine.

The measuring point 12 may comprise a connection coupling 32, a readableand writeable information carrier 58, and a measuring pointcommunication interface 56.

The writeable information carrier 58, and the measuring pointcommunication interface 56 may be provided in a separate device 59placed in the vicinity of the stud 30, as illustrated in FIG. 2.Alternatively the writeable information carrier 58, and the measuringpoint communication interface 56 may be provided within the stud 30.This is described in more detail in WO 98/01831, the content of which ishereby incorporated by reference.

The system 2 is arranged to allow bidirectional communication betweenthe measuring point communication interface 56 and the analysisapparatus communication interface 54. The measuring point communicationinterface 56 and the analysis apparatus communication interface 54 arepreferably constructed to allow wireless communication. According to anembodiment the measuring point communication interface and the analysisapparatus communication interface are constructed to communicate withone another by radio frequency (RF) signals. This embodiment includes anantenna in the measuring point communication interface 56 and anotherantenna the analysis apparatus communication interface 54.

FIG. 4 is a simplified illustration of an embodiment of the memory 60and its contents. The simplified illustration is intended to conveyunderstanding of the general idea of storing different program functionsin memory 60, and it is not necessarily a correct technical teaching ofthe way in which a program would be stored in a real memory circuit. Thefirst memory segment 70 stores program code for controlling the analysisapparatus 14 to perform basic operations. Although the simplifiedillustration of FIG. 4 shows pseudo code, it is to be understood thatthe program code 80 may be constituted by machine code, or any levelprogram code that can be executed or interpreted by the data processingmeans 50 (FIG. 2A).

The second memory segment 90, illustrated in FIG. 4, stores a second setof program code 94. The program code 94 in segment 90, when run on thedata processing means 50, will cause the analysis apparatus 14 toperform a function, such as a digital signal processing function. Thefunction may comprise an advanced mathematical processing of the digitalmeasurement data signal S_(MD). According to embodiments of theinvention the program code 94 is adapted to cause the processor means 50to perform signal processing functions described in connection withFIGS. 5, 6, 9 and/or FIG. 16 in this document.

As mentioned above in connection with FIG. 1, a computer program forcontrolling the function of the analysis apparatus may be downloadedfrom the server computer 20. This means that theprogram-to-be-downloaded is transmitted to over the communicationsnetwork 18. This can be done by modulating a carrier wave to carry theprogram over the communications network 18. Accordingly the downloadedprogram may be loaded into a digital memory, such as memory 60 (SeeFIGS. 2A and 4). Hence, a signal processing program 94 and or ananalysis function program 104, 105 may be received via a communicationsport, such as port 16 (FIGS. 1 & 2A), so as to load it into memory 60.Similarly, a signal processing program 94 and or an analysis functionprogram 104, 105 may be received via communications port 29B (FIG. 1),so as to load it into a program memory location in computer 26B or indatabase 22B.

An aspect of the invention relates to a computer program product, suchas a program code means 94 and/or program code means 104, 105 loadableinto a digital memory of an apparatus. The computer program productcomprising software code portions for performing signal processingmethods and/or analysis functions when said product is run on a dataprocessing unit 50 of an apparatus for analysing the condition of amachine. The term “run on a data processing unit” means that thecomputer program plus the data processing unit carries out a method ofthe kind described in this document.

The wording “a computer program product, loadable into a digital memoryof a condition analysing apparatus” means that a computer program can beintroduced into a digital memory of a condition analysing apparatus soas achieve a condition analysing apparatus programmed to be capable of,or adapted to, carrying out a method of the kind described above. Theterm “loaded into a digital memory of a condition analysing apparatus”means that the condition analysing apparatus programmed in this way iscapable of, or adapted to, carrying out a method of the kind describedabove.

The above mentioned computer program product may also be loadable onto acomputer readable medium, such as a compact disc or DVD. Such a computerreadable medium may be used for delivery of the program to a client.

According to an embodiment of the analysis apparatus 14 (FIG. 2A), itcomprises a user input interface 102, whereby an operator may interactwith the analysis apparatus 14. According to an embodiment the userinput interface 102 comprises a set of buttons 104. An embodiment of theanalysis apparatus 14 comprises a user output interface 106. The useroutput interface may comprise a display unit 106. The data processingmeans 50, when it runs a basic program function provided in the basicprogram code 80, provides for user interaction by means of the userinput interface 102 and the display unit 106. The set of buttons 104 maybe limited to a few buttons, such as for example five buttons, asillustrated in FIG. 2A. A central button 107 may be used for an ENTER orSELECT function, whereas other, more peripheral buttons may be used formoving a cursor on the display 106. In this manner it is to beunderstood that symbols and text may be entered into the apparatus 14via the user interface. The display unit 106 may, for example, display anumber of symbols, such as the letters of alphabet, while the cursor ismovable on the display in response to user input so as to allow the userto input information.

FIG. 5 is a schematic block diagram of an embodiment of the analysisapparatus 14 at a client location 4 with a machine 6 having a movableshaft 8. The sensor 10, which may be a Shock Pulse Measurement Sensor,is shown attached to the body of the machine 6 so as to pick upmechanical vibrations and so as to deliver an analogue measurementsignal S_(EA) indicative of the detected mechanical vibrations to thesensor interface 40. The sensor interface 40 may be designed asdescribed in connection with FIG. 2A or 2B. The sensor interface 40delivers a digital measurement data signal S_(MD) to a means 180 fordigital signal processing.

The digital measurement data signal S_(MD) has a sampling frequencyf_(S), and the amplitude value of each sample depends on the amplitudeof the received analogue measurement signal S_(EA) at the moment ofsampling. According to an embodiment the sampling frequency f_(S) of thedigital measurement data signal S_(MD) may be fixed to a certain valuef_(S), such as e.g. f_(S)=102 kHz. The sampling frequency f_(S) may becontrolled by a clock signal delivered by a clock 190, as illustrated inFIG. 5. The clock signal may also be delivered to the means 180 fordigital signal processing. The means 180 for digital signal processingcan produce information about the temporal duration of the receiveddigital measurement data signal S_(MD) in response to the receiveddigital measurement data signal S_(MD), the clock signal and therelation between the sampling frequency f_(S) and the clock signal,since the duration between two consecutive sample values equalsT_(S)=1/f_(S).

According to embodiments of the invention the means 180 for digitalsignal processing includes a pre-processor 200 for performing apre-processing of the digital measurement data signal S_(MD) so as todeliver a pre-processed digital signal S_(MDP) on an output 210. Theoutput 210 is coupled to an input 220 of an evaluator 230. The evaluator230 is adapted to evaluate the pre-processed digital signal S_(MDP) soas to deliver a result of the evaluation to a user interface 106.Alternatively the result of the evaluation may be delivered to acommunication port 16 so as to enable the transmission of the resulte.g. to a control computer 33 at a control site 31 (See FIG. 1).

According to an embodiment of the invention, the functions described inconnection with the functional blocks in means 180 for digital signalprocessing, pre-processor 200 and evaluator 230 may be embodied bycomputer program code 94 and/or 104 as described in connection withmemory blocks 90 and 100 in connection with FIG. 4 above.

A user may require only a few basic monitoring functions for detectionof whether the condition of a machine is normal or abnormal. Ondetecting an abnormal condition, the user may call for specializedprofessional maintenance personnel to establish the exact nature of theproblem, and for performing the necessary maintenance work. Theprofessional maintenance personnel frequently needs and uses a broadrange of evaluation functions making it possible to establish the natureof, and/or cause for, an abnormal machine condition. Hence, differentusers of an analysis apparatus 14 may pose very different demands on thefunction of the apparatus. The term Condition Monitoring function isused in this document for a function for detection of whether thecondition of a machine is normal or somewhat deteriorated or abnormal.The term Condition Monitoring function also comprises an evaluationfunction making it possible to establish the nature of, and/or causefor, an abnormal machine condition.

Examples of Machine Condition Monitoring Functions

The condition monitoring functions F1, F2 . . . Fn includes functionssuch as: vibration analysis, temperature analysis, shock pulsemeasuring, spectrum analysis of shock pulse measurement data, FastFourier Transformation of vibration measurement data, graphicalpresentation of condition data on a user interface, storage of conditiondata in a writeable information carrier on said machine, storage ofcondition data in a writeable information carrier in said apparatus,tachometering, imbalance detection, and misalignment detection.

According to an embodiment the apparatus 14 includes the followingfunctions:

F1=vibration analysis;

F2=temperature analysis,

F3=shock pulse measuring,

F4=spectrum analysis of shock pulse measurement data,

F5=Fast Fourier Transformation of vibration measurement data,

F6=graphical presentation of condition data on a user interface,

F7=storage of condition data in a writeable information carrier on saidmachine,

F8=storage of condition data in a writeable information carrier 52 insaid apparatus,

F9=tachometering,

F10=imbalance detection, and

F11=misalignment detection.

F12=Retrieval of condition data from a writeable information carrier 58on said machine.

F13=Performing vibration analysis function F1 and performing functionF12 “Retrieval of condition data from a writeable information carrier 58on said machine” so as to enable a comparison or trending based oncurrent vibration measurement data and historical vibration measurementdata.F14=Performing temperature analysis F2; and performing function“Retrieval of condition data from a writeable information carrier 58 onsaid machine” so as to enable a comparison or trending based on currenttemperature measurement data and historical temperature measurementdata.F15=Retrieval of identification data from a writeable informationcarrier 58 on said machine.

Embodiments of the function F7 “storage of condition data in a writeableinformation carrier on said machine”, and F13 vibration analysis andretrieval of condition data is described in more detail in WO 98/01831,the content of which is hereby incorporated by reference.

FIG. 6 illustrates a schematic block diagram of an embodiment of thepre-processor 200 according to an embodiment of the present invention.In this embodiment the digital measurement data signal S_(MD) is coupledto a digital band pass filter 240 having a lower cutoff frequencyf_(LC), an upper cutoff frequency f_(UC) and passband bandwidth betweenthe upper and lower cutoff frequencies.

The output from the digital band pass filter 240 is connected to adigital enveloper 250. According to an embodiment of the invention thesignal output from the enveloper 250 is delivered to an output 260. Theoutput 260 of the pre-processor 200 is coupled to output 210 of digitalsignal processing means 180 for delivery to the input 220 of evaluator230.

The upper and lower cutoff frequencies of the digital band pass filter240 may selected so that the frequency components of the signal S_(MD)at the resonance frequency f_(RM) for the sensor are in the passbandbandwidth. As mentioned above, an amplification of the mechanicalvibration is achieved by the sensor being mechanically resonant at theresonance frequency f_(RM). Accordingly the analogue measurement signalS_(EA) reflects an amplified value of the vibrations at and around theresonance frequency f_(RM). Hence, the band pass filter according to theFIG. 6 embodiment advantageously suppresses the signal at frequenciesbelow and above resonance frequency f_(RM), so as to further enhance thecomponents of the measurement signal at the resonance frequency f_(RM).Moreover, the digital band pass filter 240 advantageously furtherreduces noise inherently included in the measurement signal, since anynoise components below the lower cutoff frequency f_(LC), and aboveupper cutoff frequency f_(UC) are also eliminated or reduced. Hence,when using a resonant Shock Pulse Measurement sensor 10 having amechanical resonance frequency f_(RM) in a range from a lowest resonancefrequency value f_(RML) to a highest resonance frequency value f_(RMU)the digital band pass filter 240 may be designed to having a lowercutoff frequency f_(LC)=f_(RML), and an upper cutoff frequencyf_(UC)=f_(RMU). According to an embodiment the lower cutoff frequencyf_(LC)=f_(RML)=28 kHz, and the upper cutoff frequency f_(UC)=f_(RMU)=37kHz.

According to another embodiment the mechanical resonance frequencyf_(RM) is somewhere in the range from 30 kHz to 35 kHz, and the digitalband pass filter 240 may then be designed to having a lower cutofffrequency f_(LC)=30 kHz and an upper cutoff frequency f_(UC)=35 kHz.

According to another embodiment the digital band pass filter 240 may bedesigned to have a lower cutoff frequency f_(LC) being lower than thelowest resonance frequency value f_(RM), and an upper cutoff frequencyf_(UC) being higher than the highest resonance frequency value f_(RMU).For example the mechanical resonance frequency f_(RM) may be a frequencyin the range from 30 kHz to 35 kHz, and the digital band pass filter 240may then be designed to having a lower cutoff frequency f_(LC)=17 kHz,and an upper cutoff frequency f_(UC)=36 kHz.

Accordingly the digital band pass filter 240 delivers a passband digitalmeasurement data signal S_(F) having an advantageously low noise contentand reflecting mechanical vibrations in the passband. The passbanddigital measurement data signal S_(F) is delivered to enveloper 250.

The digital enveloper 250 accordingly receives the passband digitalmeasurement data signal S_(F) which may reflect a signal having positiveas well as negative amplitudes. With reference to FIG. 6, the receivedsignal is rectified by a digital rectifier 270, and the rectified signalmay be filtered by an optional low pass filter 280 so as to produce adigital envelop signal S_(ENV).

Accordingly, the signal S_(ENV) is a digital representation of anenvelope signal being produced in response to the filtered measurementdata signal S_(F). According to some embodiments of the invention theoptional low pass filter 280 may be eliminated. One such embodiment isdiscussed in connection with FIG. 9 below. Accordingly, the optional lowpass filter 280 in enveloper 250 may be eliminated when decimator 310,discussed in connection with FIG. 9 below, includes a low pass filterfunction.

According to the FIG. 6 embodiment of the invention the signal S_(ENV)is delivered to the output 260 of pre-processor 200. Hence, according toan embodiment of the invention the pre-processed digital signal S_(MDP)delivered on the output 210 (FIG. 5) is the digital envelop signalS_(ENV).

Whereas prior art analogue devices for generating an envelop signal inresponse to a measurement signal employs an analogue rectifier whichinherently leads to a biasing error being introduced in the resultingsignal, the digital enveloper 250 will advantageously produce a truerectification without any biasing errors. Accordingly, the digitalenvelop signal S_(ENV) will have a good Signal-to-Noise Ratio, since thesensor being mechanically resonant at the resonance frequency in thepassband of the digital band pass filter 240 leads to a high signalamplitude and the signal processing being performed in the digitaldomain eliminates addition of noise and eliminates addition of biasingerrors.

With reference to FIG. 5 the pre-processed digital signal S_(MDP) isdelivered to input 220 of the evaluator 230.

According to another embodiment, the filter 240 is a high pass filterhaving a cut-off frequency f_(LC). This embodiment simplifies the designby replacing the band-pass filter with a high-pass filter 240, therebyleaving the low pass filtering to another low pass filter downstream,such as the low pass filter 280. The cut-off frequency f_(LC) of thehigh pass filter 240 is selected to approximately the value of thelowest expected mechanical resonance frequency value f_(RMU) of theresonant Shock Pulse Measurement sensor 10. When the mechanicalresonance frequency f_(RM) is somewhere in the range from 30 kHz to 35kHz, the high pass filter 240 may be designed to having a lower cutofffrequency f_(LC)=30 kHz. The high-pass filtered signal is then passed tothe rectifier 270 and on to the low pass filter 280.

According to an embodiment it should be possible to use sensors 10having a resonance frequency somewhere in the range from 20 kHz to 35kHz. In order to achieve this, the high pass filter 240 may be designedto having a lower cutoff frequency f_(LC)=20 kHz.

FIG. 7 illustrates an embodiment of the evaluator 230 (See also FIG. 5).The FIG. 7 embodiment of the evaluator 230 includes a condition analyser290 adapted to receive a pre-processed digital signal S_(MDP) indicativeof the condition of the machine 6. The condition analyser 290 can becontrolled to perform a selected condition analysis function by means ofa selection signal delivered on a control input 300. The selectionsignal delivered on control input 300 may be generated by means of userinteraction with the user interface 102 (See FIG. 2A). When the selectedanalysis function includes Fast Fourier Transform, the analyzer 290 willbe set by the selection signal 300 to operate on an input signal in thefrequency domain.

Dependent on what type of analysis to be performed the conditionanalyser 290 may operate on an input pre-processed digital signalS_(MDP) in the time domain, or on an input pre-processed digital signalS_(MDP) in the frequency domain. Accordingly, dependent on the selectionsignal delivered on control input 300, the FFT 294 may be included asshown in FIG. 8, or the signal S_(MDP) may be delivered directly to theanalyser 290 as illustrated in FIG. 7.

FIG. 8 illustrates another embodiment of the evaluator 230. In the FIG.8 embodiment the evaluator 230 includes an optional Fast FourierTransformer 294 coupled to receive the signal from input 220 of theevaluator 230. The output from the FFTransformer 294 may be delivered toanalyser 290.

In order to analyze the condition of a rotating part it is desired tomonitor the detected vibrations for a sufficiently long time to be ableto detect repetitive signals. Certain repetitive signal signatures areindicative of a deteriorated condition of the rotating part. An analysisof a repetitive signal signature may also be indicative of the type ofdeteriorated condition. Such an analysis may also result in detection ofthe degree of deteriorated condition.

Hence, the measurement signal may include at least one vibration signalcomponent S_(D) dependent on a vibration movement of the rotationallymovable part 8; wherein said vibration signal component has a repetitionfrequency f_(D) which depends on the speed of rotation f_(ROT) of therotationally movable part 8. The vibration signal component which isdependent on the vibration movement of the rotationally movable part 8may therefore be indicative of a deteriorated condition or a damage ofthe monitored machine. In fact, a relation between repetition frequencyf_(D) of the vibration signal component S_(D) and the speed of rotationf_(ROT) of the rotationally movable part 8 may be indicative of whichmechanical part it is that has a damage. Hence, in a machine having aplurality of rotating parts it may be possible to identify an individualslightly damaged part by means of processing the measurement signalusing an analysis function 105, including a frequency analysis.

Such a frequency analysis may include fast fourier transformation of themeasurement signal including vibration signal component S_(D). The fastfourier transformation (FFT), uses a certain frequency resolution. Thatcertain frequency resolution, which may be expressed in terms offrequency bins, determines the limit for discerning differentfrequencies. The term “frequency bins” is sometimes referred to as“lines”. If a frequency resolution providing Z frequency bins up to theshaft speed is desired, then it is necessary to record the signal duringX revolutions of the shaft.

In connection with the analysis of rotation parts it may be interestingto analyse signal frequencies that are higher than the rotationfrequency f_(ROT) of the rotating part. The rotating part may include ashaft and bearings. The shaft rotation frequency f_(ROT) is oftenreferred to as “order 1”. The interesting bearing signals may occurabout ten times per shaft revolution (Order 10), i.e. a damagerepetition frequency f_(D) (measured in Hz) divided by rotational speedf_(ROT) (measured in rps) equals 10 Hz/rps, i.e. ordery=f_(D)/f_(ROT)=10 Hz/rps. Moreover, it may be interesting to analyseovertones of the bearing signals, so it may be interesting to measure upto order 100. Referring to a maximum order as Y, and the total number offrequency bins in the FFT to be used as Z, the following applies: Z=X*Y.Conversely, X=Z/Y, wherein

-   -   X is the number of revolutions of the monitored shaft during        which the digital signal is analysed; and    -   Y is a maximum order; and    -   Z is the frequency resolution expressed as a number of frequency        bins

Consider a case when the decimated digital measurement signal S_(MDP)(See FIG. 5) is delivered to the FFT analyzer 294, as described in FIG.8: In such a case, when the FFT analyzer 294 is set for Z=1600 frequencybins, and the user is interested in analysing frequencies up to orderY=100, then the value for X becomes X=Z/Y=1600/100=16.

Hence, it is necessary to measure during X=16 shaft revolutions whenZ=1600 frequency bins is desired and the user is interested in analysingfrequencies up to order Y=100.

The frequency resolution Z of the FFT analyzer 294 may be settable usingthe user interface 102, 106 (FIG. 2A).

Hence, the frequency resolution value Z for the condition analysisfunction 105 and/or signal processing function 94 (FIG. 4) may besettable using the user interface 102, 106 (FIG. 2A).

According to an embodiment of the invention, the frequency resolution Zis settable by selecting one value Z from a group of values. The groupof selectable values for the frequency resolution Z may include

Z=400

Z=800

Z=1600

Z=3200

Z=6400

As mentioned above, the sampling frequency f_(S) may be fixed to acertain value such as e.g f_(S)=102 400 kHz, and the factor k may be setto 2.56, thereby rendering the maximum frequency to be analyzedf_(SEAmax) to be:f _(SEAmax) =f _(S) /k=102 400/2.56=40 kHz

For a machine having a shaft with rotational speed f_(ROT)=1715rpm=28.58 rps, a selected order value Y=100 renders a maximum frequencyto be analyzed to bef _(ROT) *Y=28.58 rps*100=2858 Hz.

The FFTransformer 294 may be adapted to perform Fast Fourier Transformon a received input signal having a certain number of sample values. Itis advantageous when the certain number of sample values is set to aneven integer which may be divided by two (2) without rendering afractional number.

Accordingly, a data signal representing mechanical vibrations emanatingfrom rotation of a shaft may include repetitive signal patterns. Acertain signal pattern may thus be repeated a certain number of timesper revolution of the shaft being monitored. Moreover, repetitivesignals may occur with mutually different repetition frequency.

In the book “Machinery Vibration Measurements and Analysis” by VictorWowk (ISBN 0-07-071936-5), there is provided a couple of examples ofmutually different repetition frequencies on page 149:

“Fundamental train frequency (FTF)

Ball spin (BS) frequency

Outer Race (OR)

Inner Race (IR)”

The book also provides formulas for calculating these specificfrequencies on page 150. The content of the book “Machinery VibrationMeasurements and Analysis” by Victor Wowk, is hereby incorporated byreference. In particular the above mentioned formulas for calculatingthese specific frequencies are hereby incorporated by reference. A tableon page 151 of the same book indicates that these frequencies also varydependent on bearing manufacturer, and that

-   -   FTF may have a bearing frequency factor of 0.378;    -   BS may have a bearing frequency factor of 1.928;    -   OR may have a bearing frequency factor of 3.024; and    -   IR may have a bearing frequency factor of 4.976

The frequency factor is multiplied with the rotational speed of theshaft to obtain the repetition frequency. The book indicates that for ashaft having a rotational speed of 1715 rpm, i.e. 28.58 Hz, therepetition frequency for a pulse emanating from the Outer Race (OR) of abearing of standard type 6311 may be about 86 Hz.; and the FTFrepetition frequency may be 10.8 Hz.

When the monitored shaft rotates at a constant rotational speed such arepetition frequency may be discussed either in terms of repetition pertime unit or in terms of repetition per revolution of the shaft beingmonitored, without distinguishing between the two. However, if themachine part rotates at a variable rotational speed the matter isfurther complicated, as discussed below in connection with FIGS. 16, 17and 20.

Machinery Presenting Sudden Damages

Some types of machinery may suffer complete machine failure or breakdownvery abruptly. For some machine types, such as rotating parts in a windpower station, breakdown has been known to occur suddenly and as acomplete surprise to the maintenance personnel and to the machine owner.Such sudden breakdown causes a lot of costs to the machine owner and maycause other negative side effects e.g. if machine parts fall off as aresult of unexpected mechanical failure.

The inventor realized that there is a particularly high noise level inthe mechanical vibrations of certain machinery, and that such noiselevels hamper the detection of machine damages. Hence, for some types ofmachinery, conventional methods for preventive condition monitoring havefailed to provide sufficiently early and/or reliable warning ofon-coming deteriorating conditions. The inventor concluded that theremay exist a mechanical vibration V_(MD) indicative of a deterioratedcondition in such machinery, but that conventional methods for measuringvibrations may hitherto have been inadequate.

The inventor also realized that machines having slowly rotating partswere among the types of machinery that seem to be particularly prone tosudden failure.

Having realized that a particularly high noise level in the mechanicalvibrations of certain machinery hampers the detection of machinedamages, the inventor came up with a method for enabling detection ofweak mechanical signals in a noisy environment. As mentioned above, therepetition frequency f_(D) of vibration signal component S_(D) inmeasuring signal S_(EA) depends on a mechanical vibration V_(MD) whichis indicative of an incipient damage of a rotational part 8 of themonitored machine 6. The inventor realized that it may be possible todetect an incipient damage, i.e. a damage that is just starting todevelop, if a corresponding weak signal can be discerned.

Hence, the measurement signal may include at least one vibration signalcomponent S_(D) dependent on a vibration movement of the rotationallymovable part 8; wherein said vibration signal component has a repetitionfrequency f_(D) which depends on the speed of rotation f_(ROT) of therotationally movable part 8. The existence of a vibration signalcomponent which is dependent on the vibration movement of therotationally movable part 8 may therefore provide an early indication ofa deteriorating condition or an incipient damage of the monitoredmachine.

In a wind turbine application the shaft whose bearing is analyzed mayrotate at a speed of less than 120 revolutions per minute, i.e. theshaft rotational frequency f_(ROT) is less than 2 revolutions per second(rps). Sometimes such a shaft to be analyzed rotates at a speed of lessthan 50 revolutions per minute (rpm), i.e. a shaft rotational frequencyf_(ROT) of less than 0.83 rps. In fact the speed of rotation maytypically be less than 15 rpm. Whereas a shaft having a rotational speedof 1715 rpm, as discussed in the above mentioned book, produces 500revolutions in just 17.5 seconds; a shaft rotating at 50 revolutions perminute takes ten minutes to produce 500 revolutions. Certain large windpower stations have shafts that may typically rotate at 12 RPM=0.2 rps.

Accordingly, when a bearing to be analyzed is associated with a slowlyrotating shaft, and the bearing is monitored by a detector generating ananalogue measurement signal S_(EA) which is sampled using a samplingfrequency f_(S) of about 100 Khz, the number of sampled valuesassociated with one full revolution of the shaft becomes very large. Asan illustrative example, it takes 60 million (60 000 000) sample valuesat a sampling frequency of 100 kHz to describe 500 revolutions when theshaft rotates at 50 rpm.

Moreover, performing advanced mathematical analysis of the signalrequires a lot of time when the signal includes so many samples.Accordingly it is desired to reduce the number of samples per secondbefore further processing of the signal S_(ENV).

FIG. 9 illustrates another embodiment of the pre-processor 200. The FIG.9 embodiment of the pre-processor 200 includes a digital band passfilter 240 and a digital enveloper 250 as described above in connectionwith FIG. 6. As mentioned above, the signal S_(ENV) is a digitalrepresentation of an enveloped signal which is produced in response tothe filtered measurement data signal S_(F).

According to the FIG. 9 embodiment of the pre-processor 200, the digitalenveloped signal S_(ENV) is delivered to a decimator 310 adapted toproduce a digital signal S_(RED) having a reduced sampling frequencyf_(SR1). The decimator 310 operates to produce an output digital signalwherein the temporal duration between two consecutive sample values islonger than the temporal duration between two consecutive sample valuesin the input signal. The decimator is described in more detail inconnection with FIG. 14, below. According to an embodiment of theinvention the optional low pass filter 280 may be eliminated, asmentioned above. When, in the FIG. 9 embodiment, the signal produced bythe digital rectifier 270 is delivered to decimator 310, which includeslow pass filtering, the low pass filter 280 may be eliminated.

An output 312 of the decimator 310 delivers the digital signal S_(RED)to an input 315 of an enhancer 320. The enhancer 320 is capable ofreceiving the digital signal S_(RED) and in response thereto generatingan output signal S_(MDP). The output signal S_(MDP) is delivered tooutput port 260 of pre-processor 200.

FIG. 10A is a flow chart that illustrates embodiments of a method forenhancing repetitive signal patterns in signals. This method mayadvantageously be used for enhancing repetitive signal patterns insignals representing the condition of a machine having a rotating shaft.An enhancer 320 may be designed to operate according to the methodillustrated by FIG. 10A.

Method steps S1000 to S1040 in FIG. 10A represent preparatory actions tobe taken in order to make settings before actually generating the outputsignal values. Once, the preparatory actions have been executed, theoutput signal values may be calculated, as described with reference tostep S1050.

FIG. 10B is a flow chart illustrating a method of generating a digitaloutput signal. More particularly, FIG. 10B illustrates an embodiment ofa method to generate a digital output signal when preparatory actionsdescribed with reference to steps S1000 to S1040 in FIG. 10A have beenperformed.

With reference to step S1000 in FIG. 10A, a desired length O_(LENGTH) ofan output signal S_(MDP) is determined.

FIG. 11 is a schematic illustration of a first memory having pluralmemory positions i. The memory positions i of the first memory hold anexample input signal I comprising a sequence of digital values. Theexample input signal is used for calculating the output signal S_(MDP)according to embodiments of the invention. FIG. 11 shows some of manyconsecutive digital values for the input signal I. The digital values2080 in the input signal I only illustrate a few of the digital valuesthat are present in the input signal. In FIG. 11 two neighbouringdigital values in the input signal are separated by a durationt_(delta). The value t_(delta) is the inverse of a sampling frequencyf_(SR) of the input signal received by the enhancer 320 (See FIG. 9 &FIG. 16).

FIG. 12 is a schematic illustration of a second memory having pluralmemory positions t. The memory positions t of the second memory hold anexample output signal S_(MDP) comprising a sequence of digital values.Hence, FIG. 12 illustrates a portion of a memory having digital values3090 stored in consecutive memory positions. FIG. 12 shows consecutivedigital values for the output signal S_(MDP). The digital values 3090 inthe output signal S_(MDP) only illustrate a few of the digital valuesthat are present in the output signal. In FIG. 12 two neighbouringdigital values in the output signal may be temporally separated by theduration t_(delta).

With reference to step S1000 in FIG. 10, the desired length O_(LENGTH)3010 of the output signal S_(MDP) may be chosen so that it is possibleto use the output signal S_(MDP) for analysing certain frequencies inthe output signal. If for instance lower frequencies are of interest alonger output signal is required than if higher frequencies are ofinterest. The lowest frequency that can be analysed using the outputsignal is 1/(O_(LENGTH)*t_(delta)), where O_(LENGTH) is the number ofsample values in the output signal. If f_(SR) is the sampling rate ofthe input signal I, then the time t_(delta) between each digital samplevalue will be 1/f_(SR). As mentioned above, repetitive signal patternsmay occur in a data signal representing mechanical vibrations.Accordingly, a measurement signal, such as signal S_(ENV) delivered bythe enveloper 250 and signal S_(RED) delivered to enhancer 320 mayinclude at least one vibration signal component S_(D) dependent on avibration movement of the rotationally movable part 8; wherein saidvibration signal component S_(D) has a repetition frequency f_(D) whichdepends on the speed of rotation f_(ROT) of the rotationally movablepart 8. Hence, in order to be certain to detect the occurrence of arepetitive signal pattern having a repetition frequencyf_(REP)=f_(D)=1/(O_(LENGTH)*t_(delta)) the output signal S_(MDP) mustinclude at least O_(LENGTH) digital values, when consecutive digitalvalues in the output signal S_(MDP) are separated by the durationt_(delta).

According to an embodiment, the user may input a value representing alowest repetition frequency f_(REPmin) to be detected as well asinformation about a lowest expected speed of rotation of the shaft to bemonitored. The analysis system 2 (FIG. 1) includes functionality forcalculating a suitable value for the variable O_(LENGTH) in response tothese values.

Alternatively, with reference to FIG. 2A, a user of an analysisapparatus 14 may set the value O_(LENGTH) 3010 of the output signalS_(MDP) by means of inputting a corresponding value via the userinterface 102.

In a next step S1010 a length factor L is chosen. The length factor Ldetermines how well stochastic signals are suppressed in the outputsignal S_(MDP). A higher value of L gives less stochastic signals in theoutput signal S_(MDP) than a lower value of L. Hence, the length factorL may be referred to as a Signal-Noise Ratio improver value. Accordingto one embodiment of the method L is an integer between 1 and 10, but Lcan also be set to other values. According to an embodiment of themethod, the value L can be preset in the enhancer 320. According toanother embodiment of the method the value L is inputted by a user ofthe method through the user interface 102 (FIG. 2A). The value of thefactor L also has an impact on calculation time required to calculatethe output signal. A larger value of L requires longer calculation timethan a lower value of L.

Next, in a step S1020, a starting position S_(START) is set. Thestarting position S_(START) is a position in the input signal I.

The starting position S_(START) is set to avoid or reduce the occurrenceof non-repetitive patterns in the output signal S_(MDP). When thestarting position S_(START) is set so that a part 2070 of the inputsignal before the starting position has a length which corresponds to acertain time interval T_(STOCHASTIC) _(_) _(MAX) then stochastic signalswith the a corresponding frequency f_(STOCHASTIC) _(_) _(MAX) and higherfrequencies will be attenuated in the output signal O, S_(MDP).

In a next step S1030 the required length of the input data signal iscalculated. The required length of the input data signal is calculatedin the step S1030 according to formula (1) below:I _(LENGTH) =O _(LENGTH) *L+S _(START) +O _(LENGTH)  (1)

Next, in a step S1040, a length C_(LENGTH) in the input data signal iscalculated. The length C_(LENGTH) is the length over which thecalculation of the output data signal is performed. This lengthC_(LENGTH) is calculated according to formula (3) below.C _(LENGTH) =I _(LENGTH) −S _(START) −O _(LENGTH)  (3)

Formula (3) can also be written asI_(LENGTH)=C_(LENGTH)+S_(START)+O_(LENGTH)

The output signal is then calculated in a step S1050. The output signalis calculated according to formula (5) below. In formula (5) a value forthe output signal is calculated for a time value t in the output signal.

$\begin{matrix}{{{S_{MDP}(t)} = {\sum\limits_{i = 1}^{i = {CLENGTH}}{{I(i)}*{I\left( {i + {Sstart} + t} \right)}\mspace{14mu}{where}}}}{1 \leq t \leq O_{LENGTH}}} & (5)\end{matrix}$

The output signal S_(MDP) has a length O_(LENGTH), as mentioned above.To acquire the entire output signal S_(MDP) a value for each time valuefrom t=1 to t=O_(LENGTH) has to be calculated with formula (5). In FIG.11 a digital value 2081 illustrates one digital value that is used inthe calculation of the output signal. The digital value 2081 illustratesone digital value that is used in the calculation of the output signalwhere i=1. The digital value 2082 illustrates another digital value thatis used in the calculation of the output signal. Reference numeral 2082refers to the digital value I(1+S_(START)+t) in formula (5) above, wheni=1 and t=1. Hence, reference numeral 2082 illustrates the digitalsample value at position number P in the input signal:P=1+S _(START)+1=S _(START)+2.

In FIG. 12, reference numeral 3091 refers to the digital sample valueS_(MDP)(t) in the output signal where t=1.

Another embodiment of the method for operating the enhancer 320 forenhancing repetitive patterns in signals representing the condition of amachine having a rotating shaft will now be described. According to anembodiment the length O_(LENGTH) may be preset in the enhancer 320.According to other embodiments of the method the length O_(LENGTH) maybe set by user input through the user interface 102 (FIG. 2A). Accordingto a preferred embodiment of the method the variable O_(LENGTH) is setto an even integer which may be divided by two (2) without rendering afractional number. Selecting the variable O_(LENGTH) according to thisrule advantageously adapts the number of samples in the output signal sothat it is suitable for use in the optional Fast Fourier Transformer294. Hence, according to embodiments of the method the variableO_(LENGTH) may preferably be set to a number such as e.g. 1024, 2048,4096.

In a particularly advantageous embodiment the value S_(START) is set, instep S1020, so that the part 2070 of the input signal before thestarting position has the same length as the output signal 3040, i.e.S_(START)=O_(LENGTH).

As mentioned in connection with equation (1) above, the required lengthof the input data signal isI _(LENGTH) =O _(LENGTH) *L+S _(START) +O _(LENGTH)

Hence, setting S_(START)=O_(LENGTH) in eq (1) rendersI _(LENGTH) =O _(LENGTH) *L+O _(LENGTH) +O _(LENGTH) =O _(LENGTH) *L+O_(LENGTH)*2

Accordingly, the required length of the input signal can be expressed interms of the length of the output signal according to equation (6)below.I _(LENGTH)=(L+2)*O _(LENGTH)  (6)where L is the length factor discussed above, and O_(LENGTH) is thenumber of digital values in the output signal, as discussed above.

The length C_(LENGTH) can be calculated, in this embodiment of theinvention, according to formula (7) below.C _(LENGTH) =L*O _(LENGTH)  (7)

When the preparatory actions described with reference to steps S1000 toS1040 in FIG. 10A have been performed, the digital output signal may begenerated by means of a method as described with reference to FIG. 10B.According to an embodiment of the invention, the method described withreference to FIG. 10B is performed by means of a DSP 50 (FIG. 2A).

In a step S1100 (FIG. 10B) the enhancer 320 receives a digital inputsignal I having a first plurality I_(LENGTH) of sample values on aninput 315 (See FIG. 9 and/or FIG. 16). As noted above the digital inputsignal I may represent mechanical vibrations emanating from rotation ofa shaft so far as to cause occurrence of a vibration having a period ofrepetition T_(R).

The received signal values are stored (Step S1120) in an input signalstorage portion of a data memory associated with the enhancer 320.According to an embodiment of the invention the data memory may beembodied by the read/write memory 52 (FIG. 2A).

In a step S1130 the variable t, used in equation (5) above, is set to aninitial value. The initial value may be 1 (one).

In step S1140 an output sample value S_(MDP)(t) is calculated for samplenumber t. The calculation may employ the below equation:

${S_{MDP}(t)} = {\sum\limits_{i = 1}^{i = {CLENGTH}}{{I(i)}*{I\left( {i + {Sstart} + t} \right)}}}$

The resulting sample value S_(MDP)(t) is stored (Step S1150, FIG. 10B)in an output signal storage portion of the memory 52 (See FIG. 12).

In a step S1160 the process checks the value of variable t, and if thevalue of t represents a number lower than the desired number of outputsample values O_(LENGTH) a step S1160 is performed for increasing thevalue of variable t, before repeating steps S1140, S1150 and S1160.

If, in step S1160, the value of t represents a number equal to thedesired number of output sample values O_(LENGTH) a step S1180 isperformed.

In step S1180 the output signal O, S_(MDP) is delivered on output 260(See FIG. 9 and/or FIG. 16).

As mentioned above, a data signal representing mechanical vibrationsemanating from rotation of a shaft may include repetitive signalsignatures, and a certain signal signature may thus be repeated acertain number of times per revolution of the shaft being monitored.Moreover, several mutually different repetitive signal signatures mayoccur, wherein the mutually different repetitive signal signatures mayhave mutually different repetition frequency. The method for enhancingrepetitive signal signatures in signals, as described above,advantageously enables simultaneous detection of many repetitive signalsignatures having mutually different repetition frequency. Thisadvantageously enables the simultaneous detection of e.g a Bearing InnerRace damage signature and a Bearing Outer Race damage signature in asingle measuring and analysis session, as described below.

FIG. 13 is a schematic illustration of an example output signal S_(MDP)comprising two repetitive signals signatures 4010 and 4020. The outputsignal S_(MDP) may comprise more repetitive signals signatures than theones illustrated in FIG. 13, but for illustrative purpose only tworepetitive signal signatures are shown. Only some of many digital valuesfor the repetitive signals signatures 4010 and 4020 are shown in FIG.13.

In FIG. 13 the Outer Race (OR) frequency signal 4020 and the Inner Race(IR) frequency signal 4010 are illustrated. As can be seen in FIG. 13the Outer Race (OR) frequency signal 4020 has a lower frequency than theInner Race (IR) frequency signal 4010. The repetition frequency for theOuter Race (OR) frequency signal 4020 and the Inner Race (IR) frequencysignal 4010 is 1/T_(OR) respectively 1/T_(IR).

In the above described embodiments of the method of operating theenhancer 320 for enhancing repetitive signal patterns the repetitivesignal patterns are amplified when calculating the output signal in stepS1050. A higher amplification of the repetitive signal patterns isachieved if the factor L is given a higher value,

in step S1010, than if L is given a lower value. A higher value of Lmeans that a longer input signal I_(LENGTH) is required in step S1030. Alonger input signal I_(LENGTH) therefore results in a higheramplification of the repetitive signal patterns in the output signal.Hence, a longer input signal I_(LENGTH) renders the effect of betterattenuation of stochastic signals in relation to the repetitive signalpatterns in the output signal.

According to an embodiment of the invention the integer value

I_(LENGTH) may be selected in response to a desired amount ofattenuation of stochastic signals. In such an embodiment the lengthfactor L may be determined in dependence on the selected integer valueI_(LENGTH).

Now consider an exemplary embodiment of the method for operating theenhancer 320 for enhancing repetitive signal patterns where the methodis used for amplification of a repetitive signal pattern with a certainlowest frequency. In order to be able to analyse the repetitive signalpattern with the certain lowest frequency a certain length of the outputsignal is required.

As mentioned above, using a longer input data signal in the calculationof the output signal results in that the repetitive signal pattern isamplified more than if a shorter input data signal is used. If a certainamplification of the repetitive signal pattern is required it istherefore possible to use a certain length of the input signal in orderto achieve this certain amplification of the repetitive signal pattern.

To illustrate the above mentioned embodiment consider the followingexample:

A repetitive signal pattern with a lowest repetition frequency f₁ is ofinterest. In order to ensure detection of such a repetitive signal, itwill be necessary to produce an output signal capable of indicating acomplete cycle, i.e. it needs to represent a duration of T₁=1/f₁. Whenconsecutive output signal sample values are separated by a sample periodt_(delta) the minimum number of sample values in the output signal willbe O_(Lengthmin)=T₁/t_(delta).

As mentioned above, the amount of amplification of the repetitive signalwill increase with the length of the input signal.

As mentioned above, the method described with reference to FIGS. 10 to13 above operates to enhance repetitive signal signatures in a sequenceof measurement data emanating from a rotating shaft. The wording“repetitive signal signature” is to be understood as being sample values[x(t), x(t+T), x, (t+2T), . . . x(t+nT)] including an amplitudecomponent having a non-stochastic amplitude value, and wherein aduration T between these sample values is constant, as long as the shaftrotates at a constant speed of rotation. With reference to FIG. 13 it isto be understood that digital values 4010 result from enhancing pluralrepetitive signal values in the input signal I (See FIG. 11), whereinthe input signal values are separated in time by a duration T_(IR).Hence, in that case it can be deduced that the “repetitive signalsignature” relates to a damage at the inner ring of the bearingassembly, when the period of repetition T_(IR) corresponds to a ballpass rate at the inner ring. Of course this presumes knowledge of theshaft diameter and the speed of rotation. Also, when there is such a“repetitive signal signature” signal component, there may be arepetitive signal component value x such that x(t) has similar amplitudeas x(t+T) which has similar amplitude as x(t+2T), which has similaramplitude as x(t+nT)x, and so on. When there is such a “repetitivesignal signature” present in the input signal, it may advantageously bedetected using the above described method, even when the repetitivesignal signature is so weak as to generate an amplitude componentsmaller than that of the stochastic signal components.

The method described in connection with FIGS. 10-13 may be performed bythe analysis apparatus 14 when the processor 50 executes thecorresponding program code 94, as discussed in conjunction with FIG. 4above. The data processor 50 may include a central processing unit forcontrolling the operation of the analysis apparatus 14, as well as aDigital Signal Processor (DSP). The DSP may be arranged to actually runthe program code 90 for causing the analysis apparatus 14 to execute theprogram 94 causing the process described above in connections with FIGS.10-13 to be executed. The Digital Signal Processor may be e.g. of thetype TMS320C6722, manufactured by Texas Instruments. In this manner theanalysis apparatus 14 may operate to execute all signal processingfunctions 94, including filtering function 240, enveloping function 250,decimation function 310 & 470 and enhancing function 320. According toanother embodiment of the invention, the signal processing may be sharedbetween the apparatus 14 and the computer 33, as mentioned above. Hence,apparatus 14 may receive the analogue measurement signal S_(EA) andgenerate a corresponding digital signal S_(MD), and then deliver thedigital signal S_(MD) to control computer 33, allowing further signalprocessing functions 94 to be performed at the control location 31.

Decimation of Sampling Rate

As discussed above in connection with FIG. 9, it may be desirable toprovide a decimator 310 to reduce the sampling frequency of the digitalsignal before delivery to the enhancer 320. Such a decimator 310advantageously reduces the number of samples in the signal to beanalyzed, thereby reducing the amount of memory space needed for storingthe signal to be used. The decimation also enables a faster processingin the subsequent enhancer 320.

FIG. 14A illustrates a number of sample values in the signal deliveredto the input of the decimator 310, and FIG. 14B illustrates outputsample values of the corresponding time period. The signal being inputto decimator 310 may have a sampling frequency f_(S). As can be seen theoutput signal is has a reduced sample frequency f_(SR1). The decimator310 is adapted to perform a decimation of the digitally enveloped signalS_(ENV) so as to deliver a digital signal S_(RED) having a reducedsample rate f_(SR1) such that the output sample rate is reduced by aninteger factor M as compared to the input sample rate f_(S).

Hence, the output signal S_(RED) includes only every M:th sample valuepresent in the input signal S_(ENV). FIG. 14B illustrates an examplewhere M is 4, but M could be any positive integer. According to anembodiment of the invention the decimator may operate as described inU.S. Pat. No. 5,633,811, the content of which is hereby incorporated byreference.

FIG. 15A illustrates a decimator 310 according to an embodiment of theinvention. In the embodiment 310A of decimator 310 according to FIG.15A, a comb filter 400 filters and decimates the incoming signal at aratio of 16:1. That is, the output sampling rate is reduced by a firstinteger factor M1 of sixteen (M1=16) as compared to the input samplingrate. A finite impulse response (FIR) filter 401 receives the output ofthe comb filter 400 and provides another reduction of the sampling rateby a second integer factor M2. If integer factor M2=4, the FIR filter401 renders a 4:1 reduction of the sampling rate, and thereforedecimator 310A renders a total decimation of 64:1.

FIG. 15B illustrates another embodiment of the invention, whereinembodiment 310B of the decimator 310 includes a low pass filter 402,followed by a sample selector 403. The sample selector 403 is adapted topick every M:th sample out of the signal received from the low passfilter 402. The resulting signal S_(RED1) has a sample rate off_(SR1)=f_(S)/M, where f_(S) is the sample rate of received signalS_(ENV). The cutoff frequency of the low pass filter 402 is controlledby the value M.

According to one embodiment the value M is preset to a certain value.According to another embodiment the value M may be settable. Thedecimator 310 may be settable to make a selected decimation M:1, whereinM is a positive integer. The value M may be received on a port 404 ofdecimator 310.

The cut-off frequency of low pass filter 402 is f_(SR1)/(G*M) Hertz. Thefactor G may be selected to a value of two (2.0) or a value higher thantwo (2.0). According to an embodiment the value G is selected to a valuebetween 2.5 and 3. This advantageously enables avoiding aliasing. Thelow pass filter 402 may be embodied by a FIR filter.

The signal delivered by low pass filter 402 is delivered to sampleselector 403. The sample selector receives the value M on one port andthe signal from low pass filter 402 on another port, and it generates asequence of sample values in response to these inputs. The sampleselector is adapted to pick every M:th sample out of the signal receivedfrom the low pass filter 402. The resulting signal S_(RED1) has a samplerate of f_(SR1)=1/M*f_(S), where f_(S) is the sample rate of a signalS_(ENV) received on a port 405 of the decimator 310.

A Method for Compensating for Variable Shaft Speed

As mentioned above, a repetitive signal signature being present in theinput signal may advantageously be detected using the above describedmethod, even when the repetitive signal signature is so weak as togenerate an amplitude component smaller than that of the stochasticsignal components.

However, in certain applications the shaft rotational speed may vary.Performing the method described with reference to FIGS. 10-13 using aninput measurement sequence wherein the speed of shaft rotation variesleads to deteriorated quality of the resulting output signal S_(MDP).

Accordingly an object of an aspect of the invention is to achieveequally high quality of the resulting block Y when the rotational speedof the shaft varies as when the rotational speed of the shaft isconstant during the complete measuring sequence.

FIG. 16 illustrates an embodiment of the invention including a decimator310 and an enhancer 320, as described above, and a fractional decimator470.

According to an embodiment of the invention, whereas the decimator 310operates to decimate the sampling rate by M:1, wherein M is an integer,the FIG. 16 embodiment includes a fractional decimator 470 fordecimating the sampling rate by U/N, wherein both U and N are positiveintegers. Hence, the fractional decimator 470 advantageously enables thedecimation of the sampling rate by a fractional number. According to anembodiment the values for U and N may be selected to be in the rangefrom 2 to 2000. According to an embodiment the values for U and N may beselected to be in the range from 500 to 1500. According to yet anotherembodiment the values for U and N may be selected to be in the rangefrom 900 to 1100.

In the FIG. 16 embodiment the output signal from the decimator 310 isdelivered to a selector 460. The selector enables a selection of thesignal to be input to the enhancer 320. When condition monitoring ismade on a rotating part having a constant speed of rotation, theselector 460 may be set in the position to deliver the signal S_(RED)having sample frequency f_(SR1) to the input 315 of enhancer 320, andfractional decimator 470 may be disabled. When condition monitoring ismade on a rotating part having a variable speed of rotation, thefractional decimator 470 may be enabled and the selector 460 is set inthe position to deliver the signal S_(RED2) having sample frequencyf_(SR2) to the input 315 of enhancer 320.

The fractional decimator 470 has an input 480. The input 480 may becoupled to receive the signal output from decimator 310. The fractionaldecimator 470 also has an input 490 for receiving information indicativeof the rotational speed of the shaft 8.

A speed detector 420 (See FIG. 5) may be provided to deliver a signalindicative of the speed of rotation f_(ROT) of the shaft 8. The speedsignal may be received on a port 430 of the processing means 180,thereby enabling the processing means 180 to deliver that speed signalto input 490 of fractional decimator 470. The speed of rotation f_(ROT)of the shaft 8 may be provided in terms of rotations per second, i.e.Hertz (Hz).

FIG. 17 illustrates an embodiment of the fractional decimator 470enabling the alteration of the sample rate by a fractional number U/N,wherein U and N are positive integers. This enables a very accuratecontrol of the sample rate f_(SR2) to be delivered to the enhancer 320,thereby enabling a very good detection of weak repetitive signalsignatures even when the shaft speed varies.

The speed signal, received on input 490 of fractional decimator 470, isdelivered to a Fractional Number generator 500. The Fractional Numbergenerator 500 generates integer number outputs U and N on outputs 510and 520, respectively. The U output is delivered to an upsampler 530.The upsampler 530 receives the signal S_(RED) (See FIG. 16) via input480. The upsampler 530 includes a sample introductor 540 for introducingU−1 sample values between each sample value received on port 480. Eachsuch added sample value is provided with an amplitude value. Accordingto an embodiment each such added sample value is a zero (0) amplitude.

The resulting signal is delivered to a low pass filter 550 whose cut-offfrequency is controlled by the value U delivered by Fractional Numbergenerator 500. The cut-off frequency of low pass filter 550 isf_(SR2)/(K*U) Hertz. The factor K may be selected to a value of two (2)or a value higher than two (2).

The resulting signal is delivered to a Decimator 560. The Decimator 560includes a low pass filter 570 whose cutoff frequency is controlled bythe value N delivered by Fractional Number generator 500. The cut-offfrequency of low pass filter 570 is f_(SR2)/(K*N) Hertz. The factor Kmay be selected to a value of two (2) or a value higher than two (2).

The signal delivered by low pass filter 570 is delivered to sampleselector 580. The sample selector receives the value N on one port andthe signal from low pass filter 570 on another port, and it generates asequence of sample values in response to these inputs. The sampleselector is adapted to pick every N:th sample out of the signal receivedfrom the low pass filter 570. The resulting signal S_(RED2) has a samplerate of f_(SR2)=U/N*f_(SR1), where f_(SR1) is the sample rate of asignal S_(RED) received on port 480. The resulting signal S_(RED2) isdelivered on an output port 590.

The low pass filters 550 and 570 may be embodied by FIR filters. Thisadvantageously eliminates the need to perform multiplications with thezero-amplitude values introduced by sample introductor 540.

FIG. 18 illustrates another embodiment of the fractional decimator 470.The FIG. 18 embodiment advantageously reduces the amount of calculationneeded for producing the signal S_(RED2).

In the FIG. 18 embodiment the low pass filter 570 has been eliminated,so that the signal delivered by low pass filter 550 is delivereddirectly to sample selector 580. When the fractional decimator 470 isembodied by hardware the FIG. 18 embodiment advantageously reduces anamount of hardware, thereby reducing the cost of production.

When the fractional decimator 470 is embodied by software the FIG. 18embodiment advantageously reduces an amount of program code that need tobe executed, thereby reducing the load on the processor and increasingthe execution speed.

With reference to FIGS. 17 and 18, the resulting signal S_(RED2), whichis delivered on the output port of fractional decimator 470, has asample rate of f_(SR2)=U/N*f_(SR1), where f_(SR1) is the sample rate ofa signal S_(RED) received on port 480. The fractional value U/N isdependent on a rate control signal received on input port 490. Asmentioned above, the rate control signal may be a signal indicative ofthe speed of rotation of the shaft 8, which may be delivered by speeddetector 420 (See FIG. 1 and/or FIG. 5). The speed detector 420 may beembodied by an encoder, providing a pulse signal with a suitablyselected resolution so as to enable the desired accuracy of the speedsignal. In one embodiment the encoder 420 delivers a full revolutionmarker signal once per full revolution of the shaft 8. Such a revolutionmarker signal may be in the form of an electric pulse having an edgethat can be accurately detected and indicative of a certain rotationalposition of the monitored shaft 8. According to another embodiment, theencoder 420 may deliver many pulse signals per revolution of themonitored shaft, so as to enable detection of speed variations alsowithin one revolution of the shaft.

According to an embodiment, the Fractional Number generator 500 controlsthe values of U and N so that the reduced sample rate F_(SR2) has such avalue as to provide a signal S_(RED2) wherein the number of samples perrevolution of the shaft 8 is substantially constant, irrespective of anyspeed variations of the shaft 8. Accordingly: The higher the values of Uand N, the better the ability of the fractional decimator 470 at keepingthe number of sample values per revolution of the shaft 8 at a issubstantially constant value.

The fractional decimation as described with reference to FIGS. 17 and 18may be attained by performing the corresponding method steps, and thismay be achieved by means of a computer program 94 stored in memory 60,as described above. The computer program may be executed by a DSP 50.Alternatively the computer program may be executed by a FieldProgrammable Gate Array circuit (FPGA).

The method described in connection with FIGS. 10-13 and the decimationas described with reference to FIGS. 17 and 18 may be performed by theanalysis apparatus 14 when the processor 50 executes the correspondingprogram code 94, as discussed in conjunction with FIG. 4 above. The dataprocessor 50 may include a central processing unit 50 for controllingthe operation of the analysis apparatus 14, as well as a Digital SignalProcessor (DSP) 50B. The DSP 50B may be arranged to actually run theprogram code 90 for causing the analysis apparatus 14 to execute theprogram 94 causing the process described above in connections with FIGS.10-13 to be executed. According to another embodiment the processor 50Bis a Field programmable Gate Array circuit (FPGA).

FIG. 19 illustrates decimator 310 and another embodiment of fractionaldecimator 470. Decimator 310 receives the signal S_(ENV) having asampling frequency f_(S) on a port 405, and an integer M on a port 404,as described above. Decimator 310 delivers a signal S_(RED1) having asampling frequency f_(SR1) on output 312, which is coupled to input 480of fractional decimator 470A. The output sampling frequency f_(SR1) isf _(SR1) =f _(S) /Mwherein M is an integer.

Fractional decimator 470A receives the signal S_(RED1), having asampling frequency f_(SR1), as a sequence of data values S(j), and itdelivers an output signal S_(RED2) as another sequence of data valuesR(q) on its output 590.

Fractional decimator 470A may include a memory 604 adapted to receiveand store the data values S(j) as well as information indicative of thecorresponding speed of rotation f_(ROT) of the monitored rotating part.Hence the memory 604 may store each data value S(j) so that it isassociated with a value indicative of the speed of rotation of themonitored shaft at time of detection of the sensor signal S_(EA) valuecorresponding to the data value S(j).

When generating output data values R(q) the fractional decimator 470A isadapted to read data values S(j) as well as information indicative ofthe corresponding speed of rotation f_(ROT) from the memory 604.

The data values S(j) read from the memory 604 are delivered to sampleintroductor 540 for introducing U−1 sample values between each samplevalue received on port 480. Each such added sample value is providedwith an amplitude value. According to an embodiment each such addedsample value is a zero (0) amplitude.

The resulting signal is delivered to a low pass filter 550 whose cut-offfrequency is controlled by the value U delivered by Fractional Numbergenerator 500, as described above.

The resulting signal is delivered to the sample selector 580. The sampleselector receives the value N on one port and the signal from low passfilter 550 on another port, and it generates a sequence of sample valuesin response to these inputs. The sample selector is adapted to pickevery N:th sample out of the signal received from the low pass filter550. The resulting signal S_(RED2) has a sample rate off_(SR2)=U/N*f_(SR1), where f_(SR1) is the sample rate of a signalS_(RED) received on port 480. The resulting signal S_(RED2) is deliveredon output port 590.

Hence, the sampling frequency f_(SR2) for the output data values R(q) islower than input sampling frequency f_(SR1) by a factor D. D can be setto an arbitrary number larger than 1, and it may be a fractional number.According to preferred embodiments the factor D is settable to valuesbetween 1.0 to 20.0. In a preferred embodiment the factor D is afractional number settable to a value between about 1.3 and about 3.0.The factor D may be obtained by setting the integers U and N to suitablevalues. The factor D equals N divided by U:D=N/U

According to an embodiment of the invention the integers U and N aresettable to large integers in order to enable the factor D=N/U to followspeed variations with a minimum of inaccuracy. Selection of variables Uand N to be integers larger than 1000 renders an advantageously highaccuracy in adapting the output sample frequency to tracking changes inthe rotational speed of the monitored shaft. So, for example, setting Nto 500 and U to 1001 renders D=2.002.

The variable D is set to a suitable value at the beginning of ameasurement and that value is associated with a certain speed ofrotation of a rotating part to be monitored. Thereafter, during thecondition monitoring session, the fractional value D is automaticallyadjusted in response to the speed of rotation of the rotating part to bemonitored so that the signal outputted on port 590 provides asubstantially constant number of sample values per revolution of themonitored rotating part.

As mentioned above, the encoder 420 may deliver a full revolution markersignal once per full revolution of the shaft 8. Such a full revolutionmarker signal may be in the form of an electric pulse having an edgethat can be accurately detected and indicative of a certain rotationalposition of the monitored shaft 8. The full revolution marker signal,which may be referred to as an index pulse, can be produced on an outputof the encoder 420 in response to detection of a zero angle pattern onan encoding disc that rotates when the monitored shaft rotates. This canbe achieved in several ways, as is well known to the person skilled inthis art. The encoding disc may e.g. be provided with a zero anglepattern which will produce a zero angle signal with each revolution ofthe disc. The speed variations may be detected e.g. by registering a“full revolution marker” in the memory 604 each time the monitored shaftpasses the certain rotational position, and by associating the “fullrevolution marker” with a sample value s(j) received at the sameinstant. In this manner the memory 604 will store a larger number ofsamples between two consecutive full revolution markers when the shaftrotates slower, since the A/D converter delivers a constant number ofsamples f_(S) per second.

FIG. 20 is a block diagram of decimator 310 and yet another embodimentof fractional decimator 470. This fractional decimator embodiment isdenoted 470B. Fractional decimator 470B may include a memory 604 adaptedto receive and store the data values S(j) as well as informationindicative of the corresponding speed of rotation f_(ROT) of themonitored rotating part. Hence the memory 604 may store each data valueS(j) so that it is associated with a value indicative of the speed ofrotation of the monitored shaft at time of detection of the sensorsignal S_(EA) value corresponding to the data value S(j).

Fractional decimator 470B receives the signal S_(RED1), having asampling frequency f_(SR1), as a sequence of data values S(j), and itdelivers an output signal S_(RED2), having a sampling frequency f_(SR2),as another sequence of data values R(q) on its output 590.

Fractional decimator 470B may include a memory 604 adapted to receiveand store the data values S(j) as well as information indicative of thecorresponding speed of rotation f_(ROT) of the monitored rotating part.Memory 604 may store data values S(j) in blocks so that each block isassociated with a value indicative of a relevant speed of rotation ofthe monitored shaft, as described below in connection with FIG. 21.

Fractional decimator 470B may also include a fractional decimationvariable generator 606, which is adapted to generate a fractional valueD. The fractional value D may be a floating number. Hence, thefractional number can be controlled to a floating number value inresponse to a received speed value f_(ROT) so that the floating numbervalue is indicative of the speed value f_(ROT) with a certaininaccuracy. When implemented by a suitably programmed DSP, as mentionedabove, the inaccuracy of floating number value may depend on the abilityof the DSP to generate floating number values.

Moreover, fractional decimator 470B may also include a FIR filter 608.The FIR filter 608 is a low pass FIR filter having a certain low passcut off frequency adapted for decimation by a factor D_(MAX). The factorD_(MAX) may be set to a suitable value, e.g. 20,000. Moreover,fractional decimator 470B may also include a filter parameter generator610.

Operation of fractional decimator 470B is described with reference toFIGS. 21 and 22 below.

FIG. 21 is a flow chart illustrating an embodiment of a method ofoperating the decimator 310 and the fractional decimator 470B of FIG.20.

In a first step S2000, the speed of rotation F_(ROT) of the part to becondition monitored is recorded in memory 604 (FIGS. 20 & 21), and thismay be done at substantially the same time as measurement of vibrationsor shock pulses begin. According to another embodiment the speed ofrotation of the part to be condition monitored is surveyed for a periodof time. The highest detected speed F_(ROTmax) and the lowest detectedspeed F_(ROTmin) may be recorded, e.g. in memory 604 (FIGS. 20 & 21).

In step S2010, the recorded speed values are analysed, for the purposeof establishing whether the speed of rotation varies. If the speed isdetermined to be constant, the selector 460 (FIG. 16) may beautomatically set in the position to deliver the signal S_(RED) havingsample frequency f_(SR1) to the input 315 of enhancer 320, andfractional decimator 470, 470B may be disabled. If the speed isdetermined to be variable, the fractional decimator 470, 470B may beautomatically enabled and the selector 460 is automatically set in theposition to deliver the signal S_(RED2) having sample frequency f_(SR2)to the input 315 of enhancer 320.

In step S2020, the user interface 102,106 displays the recorded speedvalue f_(ROT) or speed values f_(ROTmin), f_(ROTmax), and requests auser to enter a desired order value O_(V). As mentioned above, the shaftrotation frequency f_(R) or is often referred to as “order 1”. Theinteresting signals may occur about ten times per shaft revolution(Order 10). Moreover, it may be interesting to analyse overtones of somesignals, so it may be interesting to measure up to order 100, or order500, or even higher. Hence, a user may enter an order number O_(V) usinguser interface 102.

In step S2030, a suitable output sample rate f_(SR2) is determined.According to an embodiment output sample rate f_(SR2) is set tof_(SR2)=C*O_(V)*f_(ROTmin)

wherein

-   -   C is a constant having a value higher than 2.0    -   O_(V) is a number indicative of the relation between the speed        of rotation of the monitored part and the repetition frequency        of the signal to be analysed.    -   f_(ROTmin) is a lowest speed of rotation of the monitored part        to expected during a forthcoming measurement session. According        to an embodiment the value f_(ROTmin) is a lowest speed of        rotation detected in step S2020, as described above.

The constant C may be selected to a value of 2.00 (two) or higher inview of the sampling theorem. According to embodiments of the inventionthe Constant C may be preset to a value between 2.40 and 2.70.

wherein

-   -   k is a factor having a value higher than 2.0

Accordingly the factor k may be selected to a value higher than 2.0.According to an embodiment the factor C is advantageously selected suchthat 100*C/2 renders an integer. According to an embodiment the factor Cmay be set to 2.56. Selecting C to 2.56 renders 100*C=256=2 raised to 8.

In step S2040, the integer value M is selected dependent on the detectedspeed of rotation f_(ROT) of the part to be monitored. The value of Mmay be automatically selected dependent on the detected speed ofrotation of the part to be monitored such that the intermediate reducedsampling frequency f_(SR1) will be higher than the desired output signalsampling frequency f_(SR2). The value of the reduced sampling frequencyf_(SR1) is also selected depending on how much of a variation ofrotational speed there is expected to be during the measuring session.According to an embodiment the sample rate f_(S) of the A/D convertermay be 102.4 kHz. According to an embodiment, the integer value M may besettable to a value between 100 and 512 so as to render intermediatereduced sampling frequency f_(SR1) values between 1024 Hz and 100 Hz.

In step S2050, a fractional decimation variable value D is determined.When the speed of rotation of the part to be condition monitored varies,the fractional decimation variable value D will vary in dependence onmomentary detected speed value.

According to another embodiment of steps S2040 and S2050, the integervalue M is set such that intermediate reduced sampling frequency f_(SR1)is at least as many percent higher than f_(SR2) (as determined in stepS2030 above) as the relation between highest detected speed valuef_(ROTmax) divided by the lowest detected speed value f_(ROTmin).According to this embodiment, a maximum fractional decimation variablevalue D_(MAX) is set to a value of D_(MIN)=f_(ROTmax)/f_(ROTmin), and aminimum fractional decimation variable value D_(MIN) is set to 1.0.Thereafter a momentary real time measurement of the actual speed valuef_(ROT) is made and a momentary fractional value D is set accordingly.

-   -   f_(ROT) is value indicative of a measured speed of rotation of        the rotating part to be monitored

In step S2060, the actual measurement is started, and a desired totalduration of the measurement may be determined. This duration may bedetermined in dependence on the degree of attenuation of stochasticsignals needed in the enhancer. Hence, the desired total duration of themeasurement may be set so that it corresponds to, or so that it exceeds,the duration needed for obtaining the input signal I_(LENGTH), asdiscussed above in connection with FIGS. 10A to 13. As mentioned abovein connection with FIGS. 10A to 13, a longer input signal I_(LENGTH)renders the effect of better attenuation of stochastic signals inrelation to the repetitive signal patterns in the output signal.

The total duration of the measurement may also be determined independence on a desired number of revolutions of the monitored part.

When measurement is started, decimator 310 receives the digital signalS_(ENV) at a rate f_(S) and it delivers a digital signal S_(RED1) at areduced rate f_(SR1)=f_(S)/M to input 480 of the fractional decimator.In the following the signal S_(RED1) is discussed in terms of a signalhaving sample values S(j), where j is an integer.

In step S2070, record data values S(j) in memory 604, and associate eachdata value with a speed of rotation value f_(ROT). According to anembodiment of the invention the speed of rotation value f_(ROT) is readand recorded at a rate f_(RR)=1000 times per second. The read & recordrate f_(RR) may be set to other values, dependent on how much the speedf_(ROT) of the monitored rotating part varies.

In a subsequent step S2080, analyze the recorded speed of rotationvalues, and divide the recorded data values S(j) into blocks of datadependent on the speed of rotation values. In this manner a number ofblocks of block of data values S(j) may be generated, each block of datavalues S(j) being associated with a speed of rotation value. The speedof rotation value indicates the speed of rotation of the monitored part,when this particular block data values S(j) was recorded. The individualblocks of data may be of mutually different size, i.e. individual blocksmay hold mutually different numbers of data values S(j).

If, for example, the monitored rotating part first rotated at a firstspeed f_(ROT1) during a first time period, and it thereafter changedspeed to rotate at a second speed f_(ROT2) during a second, shorter,time period, the recorded data values S(j) may be divided into twoblocks of data, the first block of data values being associated with thefirst speed value f_(ROT1), and the second block of data values beingassociated with the second speed value f_(ROT2). In this case the secondblock of data would contain fewer data values than the first block ofdata since the second time period was shorter.

According to an embodiment, when all the recorded data values S(j) havebeen divided into blocks, and all blocks have been associated with aspeed of rotation value, then the method proceeds to execute step S2090.

In step S2090, select a first block of data values S(j), and determine afractional decimation value D corresponding to the associated speed ofrotation value f_(ROT). Associate this fractional decimation value Dwith the first block of data values S(j). According to an embodiment,when all blocks have been associated with a corresponding fractionaldecimation value D, then the method proceeds to execute step S2090.Hence, the value of the fractional decimation value D is adapted independence on the speed f_(ROT).

In step S2100, select a block of data values S(j) and the associatedfractional decimation value D, as described in step S2090 above.

In step S2110, generate a block of output values R in response to theselected block of input values S and the associated fractionaldecimation value D. This may be done as described with reference to FIG.22.

In step S2120, Check if there is any remaining input data values to beprocessed. If there is another block of input data values to beprocessed, then repeat step S2100. If there is no remaining block ofinput data values to be processed then the measurement session iscompleted.

FIGS. 22A, 22B and 22C illustrate a flow chart of an embodiment of amethod of operating the fractional decimator 470B of FIG. 20.

In a step S2200, receive a block of input data values S(j) and anassociated specific fractional decimation value D. According to anembodiment, the received data is as described in step S2100 for FIG. 21above. The input data values S(j) in the received block of input datavalues S are all associated with the specific fractional decimationvalue D.

In steps S2210 to S2390 the FIR-filter 608 is adapted for the specificfractional decimation value D as received in step S2200, and a set ofcorresponding output signal values R(q) are generated. This is describedmore specifically below.

In a step S2210, filter settings suitable for the specific fractionaldecimation value D are selected. As mentioned in connection with FIG. 20above, the FIR filter 608 is a low pass FIR filter having a certain lowpass cut off frequency adapted for decimation by a factor D_(MAX). Thefactor D_(MAX) may be set to a suitable value, e.g. 20.

A filter ratio value F_(R) is set to a value dependent on factor D_(MAX)and the specific fractional decimation value D as received in stepS2200. Step S2210 may be performed by filter parameter generator 610(FIG. 20).

In a step S2220, select a starting position value x in the receivedinput data block s(j). It is to be noted that the starting positionvalue x does not need to be an integer. The FIR filter 608 has a lengthF_(LENGTH) and the starting position value x will then be selected independence of the filter length F_(LENGTH) and the filter ratio valueF_(R). The filter ratio value F_(R) is as set in step S2210 above.According to an embodiment, the starting position value x may be set tox:=F_(LENGTH)/F_(R).

In a step S2230 a filter sum value SUM is prepared, and set to aninitial value, such as e.g. SUM:=0.0

In a step S2240 a position j in the received input data adjacent andpreceding position x is selected. The position j may be selected as theinteger portion of x.

In a step S2250 select a position Fpos in the FIR filter thatcorresponds to the selected position j in the received input data. Theposition Fpos may be a fractional number. The filter position Fpos, inrelation to the middle position of the filter, may be determined to beFpos=[(x−j)*F _(R)]wherein F_(R) is the filter ratio value.

In step S2260, check if the determined filter position value Fpos isoutside of allowable limit values, i.e. points at a position outside ofthe filter. If that happens, then proceed with step S2300 below.Otherwise proceed with step S2270.

In a step S2270, a filter value is calculated by means of interpolation.It is noted that adjacent filter coefficient values in a FIR low passfilter generally have similar numerical values. Hence, an interpolationvalue will be advantageously accurate. First an integer position valueIFpos is calculated:IFpos:=Integer portion of FposThe filter value Fval for the position Fpos will be:Fval=A(IFpos)+[A(IFpos+1)−A(IFpos)]*[Fpos−Ifpos]wherein A(IFpos) and A(IFpos+1) are values in a reference filter, andthe filter position Fpos is a position between these values.

In a step S2280, calculate an update of the filter sum value SUM inresponse to signal position j:SUM:=SUM+Fval*S(j)

In a step S2290 move to another signal position:Setj:=j−1

Thereafter, go to step S2250.

In a step 2300, a position j in the received input data adjacent andsubsequent to position x is selected. This position j may be selected asthe integer portion of x, plus 1 (one), i.e j:=1+Integer portion of x

In a step S2310 select a position in the FIR filter that corresponds tothe selected position j in the received input data. The position Fposmay be a fractional number. The filter position Fpos, in relation to themiddle position of the filter, may be determined to beFpos=[(j−x)*F _(R)]wherein F_(R) is the filter ratio value.

In step S2320, check if the determined filter position value Fpos isoutside of allowable limit values, i.e. points at a position outside ofthe filter. If that happens, then proceed with step S2360 below.Otherwise proceed with step S2330.

In a step S2330, a filter value is calculated by means of interpolation.It is noted that adjacent filter coefficient values in a FIR low passfilter generally have similar numerical values. Hence, an interpolationvalue will be advantageously accurate. First an integer position valueIFpos is calculated:IFpos:=Integer portion of Fpos

The filter value for the position Fpos will be:Fval(Fpos)=A(IFpos)+[A(IFpos+1)−A(IFpos)]*[Fpos−Ifpos]wherein A(IFpos) and A(IFpos+1) are values in a reference filter, andthe filter position Fpos is a position between these values.

In a step S2340, calculate an update of the filter sum value SUM inresponse to signal position j:SUM:=SUM+Fval*S(j)

In a step S2350 move to another signal position:Setj:=j+1

Thereafter, go to step S2310.

In a step S2360, deliver an output data value R(j). The output datavalue R(j) may be delivered to a memory so that consecutive output datavalues are stored in consecutive memory positions. The numerical valueof output data value R(j) is:R(j):=SUM

In a step S2370, update position value x:x:=x+D

In a step S2380, update position value jj:=j+1

In a step S2390, check if desired number of output data values have beengenerated. If the desired number of output data values have not beengenerated, then go to step S2230. If the desired number of output datavalues have been generated, then go to step S2120 in the methoddescribed in relation to FIG. 21.

In effect, step S2390 is designed to ensure that a block of outputsignal values R(q), corresponding to the block of input data values Sreceived in step S2200, is generated, and that when output signal valuesR corresponding to the input data values S have been generated, thenstep S2120 in FIG. 21 should be executed.

The method described with reference to FIG. 22 may be implemented as acomputer program subroutine, and the steps S2100 and S2110 may beimplemented as a main program.

According to yet an embodiment of the invention, the compensation forvariable shaft speed may be achieved by controlling the clock frequencydelivered by the clock 190. As mentioned above, a speed detector 420(See FIG. 5) may be provided to deliver a signal indicative of the speedof rotation f_(ROT) of the shaft 8. The speed signal may be received ona port 430 of the processing means 180, thereby enabling the processingmeans 180 to control the clock 190. Accordingly, processing means 180may have a port 440 for delivering a clock control signal. Hence, theprocessing means 180 may be adapted to control the clock frequency inresponse to the detected speed of rotation f_(ROT).

As mentioned in connection with FIG. 2B, the sampling rate of theA/D-converter is dependent upon a clock frequency. Hence, the apparatus14 may be adapted to control the clock frequency in response to thedetected speed of rotation f_(ROT) so that the number of sample valuesper revolution of the monitored rotating part is kept at a substantiallyconstant value even when the speed of rotation varies.

According to yet another embodiment of the invention, the enhancerfunctionality 320, 94 may be achieved by a method for producingautocorrelation data as described in U.S. Pat. No. 7,010,445, thecontent of which is hereby incorporated by reference. In particular thedigital signal processor 50 may include functionality 94 for performingsuccessive Fourier Transform operations on the digitized signals toprovide autocorrelation data.

Monitoring Condition of Gear Systems

It should be noted that embodiments of the invention may also be used tosurvey, monitor and detect the condition of gear systems. Someembodiments provide particularly advantageous effects when monitoringepicyclic gear systems comprising epicyclic transmissions, gears and/orgear boxes. This will be described more in detail below. Epicyclictransmissions, gears and/or gear boxes may also be referred to asplanetary transmissions, gears and/or gear boxes.

FIG. 23 is a front view illustrating an epicyclic gear system 700. Theepicyclic gear system 700 comprises at least one or more outer gears702, 703, 704 revolving around a central gear 701. The outer gears 702,703, 704 are commonly referred to as planet gears, and the central gear701 is commonly referred to as a sun gear. The epicyclic gear system 700may also incorporate the use of an outer ring gear 705, commonly alsoreferred to as an annulus. The planet gears 702, 703, 704 may comprise Pnumber of teeth 707, the sun gear 701 may comprise S number of teeth708, and the annulus 705 may comprise A number of teeth 706. The Anumber of teeth on the annulus 705 are arranged to mesh with the Pnumber of teeth on the planet gears 702, 703, 704, which in turn arealso arranged to mesh with the S number of teeth on the sun gear 701. Itshould however be noted that the sun gear 701 is normally larger thanthe planet gears 702, 703, 704, whereby the illustration shown in FIG.23 should not be construed as limiting in this respect. When there aredifferent sizes on the sun gear 701 and the planet gears 702, 703, 704,the analysis apparatus 14 may also distinguish between detectedconditions of different shafts and gears of the epicyclic gear system700, as will become apparent from the following.

In many epicyclic gear systems, one of these three basic components,that is, the sun gear 701, the planet gears 702, 703, 704 or the annulus705, is held stationary. One of the two remaining components may thenserve as an input and provide power to the epicyclic gear system 700.The last remaining component may then serve as an output and receivepower from the epicyclic gear system 700. The ratio of input rotation tooutput rotation is dependent upon the number of teeth in each gear, andupon which component is held stationary.

FIG. 24 is a schematic side view of the epicyclic gear system 700 ofFIG. 23, as seen in the direction of the arrow SW in FIG. 23. Anexemplary arrangement 800, including the epicyclic gear system 700, maycomprise at least one sensor 10 and at least one analysis apparatus 14according to the invention as described above. The arrangement 800 may,for example, be used as gear box for wind turbines.

In an embodiment of the arrangement 800, the annulus 705 is held fixed.A rotatable shaft 801 has plural movable arms or carriers 801A, 801B,801C arranged to engage the planet gears 702, 703, 704. Upon providingan input rotation 802 to the rotatable shaft 801, the rotatable shaft801 and the movable arms 801A, 801B, 801C and the planet gears 702, 703,704 may serve as an input and provide power to the epicyclic gear system700. The rotatable shaft 801 and the planet gears 702, 703, 704 may thenrotate relative to the sun gear 701. The sun gear 701, which may bemounted on a rotary shaft 803, may thus serve as an output and receivepower from the epicyclic gear system 700. This configuration willproduce an increase in gear ratio

$G = {1 + {\frac{A}{S}.}}$As an example, the gear ratio G when used as a gear box in a windturbine may be arranged such that the output rotation is about 5-6 timesthe input rotation. The planet gears 702, 703, 704 may be mounted, viabearings 7A, 7B and 7C, respectively, on the movable arms or carriers801A, 801B and 801C (as shown in both FIGS. 23-24). The rotatable shaft801 may be mounted in bearings 7D. Similarly, the rotary shaft 803 maybe mounted in bearings 7E, and the sun gear 701 may be mounted, viabearings 7F, on the rotary shaft 803.

According to one embodiment of the invention, the at least one sensor 10may be attached on or at a measuring point 12 of the fixed annulus 705of the epicyclic gear system 700. The sensor 10 may also be arranged tocommunicate with the analysis apparatus 14. The analysis apparatus 14may be arranged to analyse the condition of the epicyclic gear system700 on the basis of measurement data or signal values delivered by thesensor 10 as described above in this document. The analysis apparatus 14may include an evaluator 230 as above.

FIG. 25 illustrates an analogue version of an exemplary signal producedby and outputted by the pre-processor 200 (see FIG. 5 or FIG. 16) inresponse to signals detected by the at least one sensor 10 upon rotationof the epicyclic gear system 700 in the arrangement 800. The signal isshown for a duration of T_(REV), which represents signal values detectedduring one revolution of the rotatable shaft 801. It is to be understoodthat the signal delivered by the pre-processor 200 on port 260 (see FIG.5 and FIG. 16) may be delivered to input 220 of the evaluator 230 (seeFIG. 8 or FIG. 7).

As can be seen from the signal in FIG. 25, the amplitude or signaloutput of the signal increases as each of the planet gears 702, 703, 704passes the measuring point 12 of the sensor 10 in the arrangement 800.These portions of the signal are referred to in the following as thehigh amplitude regions 702A, 703A, 704A, which may comprise highamplitude spikes 901. It can also be shown that the total amount ofspikes 901, 902 in the signal over one revolution of the rotatable shaft801, i.e. during the time period T_(REV), directly correlates to theamount of teeth on the annulus 705. For example, if number of teeth onthe annulus 705 is A=73, the total number of spikes in the signal duringa time period T_(REV) will be 73; or if number of teeth on the annulus705 is A=75, the total number of spikes in the signal during a timeperiod T_(REV) will be 75, etc. This has been shown to be true providedthat there are no errors or faults in the gears 702, 703, 704, 705 ofthe arrangement 800.

FIG. 26 illustrates an example of a portion of the high amplitude region702A of the signal shown in FIG. 25. This signal portion may begenerated when the planet gear 702 passes its mechanically nearestposition to the measuring point 12 and the sensor 10 (see FIGS. 23-24).It has been noted that small periodic disturbances or vibrations 903,which are illustrated in FIG. 26, may sometimes occur. Here, the smallperiodic disturbances 903 have been linked to the occurrence of errors,faults or tears in the bearings 7A, as shown in FIGS. 23-24, which maybe mounted to one of the movable arms 801A. The small periodicdisturbances 903 may thus propagate (or translate) from a bearing 7Athrough the planet gear 702 of the epicyclic gear system 700, to theannulus 705 where the small periodic disturbances 903 may be picked upby the sensor 10 as described above e.g. in connection with FIGS. 1-24.Similarly, errors, faults or tears in the bearings 7B or 7C mounted toone of the movable arms 801B or 801C may also generate such smallperiodic disturbances 903 which in the same manner as above may bepicked up by the sensor 10. It should also be noted that the smallperiodic disturbances 903 may also emanate from errors, faults or tearsin the bearings 7F which may be mounted to the rotary shaft 803. Thedetection of these small periodic disturbances in the signal may beindicative of the bearings 7A, 7B, 7C and/or 7F beginning todeteriorate, or indicative of their being on the limit of their activelifespan. This may, for example, be important since it may help predictwhen the epicyclic gear system 700 and/or the arrangement 800 are inneed of maintenance or replacement.

According to an embodiment of the invention, the condition analyser 290in the evaluator 230 of the analysis apparatus 14 may be arranged todetect these small periodic disturbances 903 in the received signal fromthe sensor 10. This is made possible by the previously describedembodiments of the invention. The small periodic disturbances 903 mayalso be referred to as shock pulses 903 or vibrations 903. According toan embodiment of the invention, the analysis apparatus 14 employing anenhancer 320 as described above enables the detection of these shockpulses 903 or vibrations 903 originating from bearings 7A (or 7B, 7C or7F) using a sensor 10 mounted on the annulus 705 as described above.Although the mechanical shock pulse or vibration signal as picked up bythe sensor 10 attached to annulus 705 may be weak, the provision of anenhancer 320 as described above makes it possible to monitor thecondition of bearings 7A (or 7B, 7C or 7F) even though the mechanicalshock pulse or vibration signal has propagated via one or several of theplanet gears 702, 703 or 704.

As previously mentioned and shown in FIGS. 7-9, the condition analyser290 may be arranged to perform suitable analysis by operating on asignal in the time domain, or a signal in the frequency domain. However,the detection of the small periodic disturbances 903 in the receivedsignal from the sensor 10 is most fittingly described in frequencydomain, as shown in FIG. 27.

FIG. 27 illustrates an exemplary frequency spectrum of a signalcomprising a small periodic disturbance 903 as illustrated in FIG. 26.The frequency spectrum of the signal comprises a peak 904 at a frequencywhich is directly correlated with the engagement or meshing of the teethof the planet gears 702, 703, 704 and the annulus 705. In fact, thefrequency of the peak 904 in the frequency spectrum will be located atA×Ω, where

-   -   A is the total number of teeth of the annulus 705, and    -   Ω is the number of revolutions per second by the rotatable shaft        801, when rotation 802 occurs at a constant speed of rotation.

In addition to the peak 904 in the frequency spectrum, the smallperiodic disturbance 903 as illustrated in FIG. 26 may generate peaks905, 906 at the frequencies f₁, f₂ centred about the peak 904 in thefrequency spectrum. The peaks 905, 906 at the frequencies f₁, f₂ maythus also be referred to as a symmetrical sideband about the centre peak904. According to an exemplary embodiment of the invention, thecondition analyser 290 may be arranged to detect the one or severalpeaks in the frequency spectrum, and thus be arranged to detect smallperiodic disturbances in the signal received from the sensor 10. It canalso be shown that the peaks 905, 906 at the frequencies f₁, f₂ relateto the centre peak 904 according to the equations Eq. 1-2:f ₁=(A×Ω)−(f _(D) ×f ₇₀₂)  (Eq. 1)f ₂=(A×Ω)+(f _(D) ×f ₇₀₂)  (Eq. 2)wherein

-   -   A is the total number of teeth of the annulus 705;    -   Ω is the number of revolutions per second by the rotatable shaft        801; and    -   f_(D) is a repetition frequency of the repetitive signal        signature which may be indicative of a deteriorated condition;        and    -   f₇₀₂ is the number of revolutions per second by the planet 702        around its own centre.

The repetition frequency f_(D) of the repetitive signal signature isindicative of the one of the rotating parts which is the origin of therepetitive signal signature. The repetition frequency f_(D) of therepetitive signal signature can also be used to distinguish betweendifferent types of deteriorated conditions, as discussed above e.g. inconnection with FIG. 8. Accordingly, a detected repetition frequencyf_(D) of the repetitive signal signature may be indicative of aFundamental train frequency (FTF), a Ball spin (BS) frequency, an OuterRace (OR) frequency, or an Inner Race (IR) frequency relating to abearing 7A, 7B, 7C or 7F in the epicyclic gear system 700 in thearrangement 800 in FIG. 24.

Hence, as described above, a data signal representing mechanicalvibrations emanating from rotation of one or several shafts, such as,rotatable shaft 801 and/or rotary shaft 803 (see FIGS. 23-24), mayinclude several repetitive signal signatures, and a certain signalsignature may thus be repeated a certain number of times per revolutionof one of the monitored shafts. Moreover, several mutually differentrepetitive signal signatures may occur, wherein the mutually differentrepetitive signal signatures may have mutually different repetitionfrequencies. The method for enhancing repetitive signal signatures insignals, as described above, advantageously enables simultaneousdetection of many repetitive signal signatures having mutually differentrepetition frequencies. This advantageously enables the simultaneousmonitoring of several bearings 7A, 7B 7C, 7F associated with differentshafts 801, 803 using a single detector 10. The simultaneous monitoringmay also use the fact that the size of the sun gear 701 and the planetgears 702, 703, 704 normally are of different sizes, which further mayenable a easy detection of which of the bearings 7A, 7B, 7C, 7F in FIGS.23-24 it is that is generating the small periodic disturbance 903, andthus which of the bearings 7A, 7B, 7C, 7F in FIGS. 23-24 may be in needof maintenance or replacement. The method for enhancing repetitivesignal signatures in signals, as described above, also advantageouslymakes it possible to distinguish between e.g. a Bearing Inner Racedamage signature and a Bearing Outer Race damage signature in a singlemeasuring and analysis session.

The relevant value for Ω, representing the speed of rotation of theplanet gears 702, 703, 704, can be indicated by a sensor 420 (see FIG.24). The sensor 420 may be adapted to generate a signal indicative ofrotation of the shaft 803 in relation to the annulus 705, and from thissignal the relevant value for Ω can be calculated when the number ofteeth of the annulus 705, the planet gears 702, 703, 704 and the sungear 701 are known.

FIG. 28 illustrates an example of a portion of the exemplary signalshown in FIG. 25. This exemplary portion demonstrates another example ofan error or fault which the condition analyser 290 also may be arrangedto detect in a similar manner as described above. If a tooth in the oneor several of the gears 701, 702, 703, 704, 705 should break or besubstantially worn down, the condition analyser 290 may be arrangeddetect that a tooth is broken or worn down since this will also generatea periodic disturbance, i.e. due to the lack of tooth engagement ormeshing of the missing or worn down tooth. This may be detectable by thecondition analyser 290 in, for example, the frequency spectrum of thesignal received from the sensor 10. It should also be noted that thistype of error or fault may be detected by the condition analyser 290 inany type of gear and/or gear system. The frequency of this type of teethengagement error, or meshing error, in a gear and/or gear system isoften located at significantly higher frequency than, for example, thefrequencies f₁, f₂ in FIG. 27.

FIG. 29 illustrates yet an embodiment of a condition analyzing system 2according to an embodiment of the invention. The sensor 10 is physicallyassociated with a machine 6 which may include a gear system 700 havingplural rotational parts (See FIG. 1 & FIG. 29). The gear system of FIG.29 may be the epicyclic gear system 700 of FIG. 24. The epicyclic gearsystem 700 may, for example, be used as gear box for wind turbines.

The sensor unit 10 may be a Shock Pulse Measurement Sensor adapted toproduce an analogue signal S_(EA) including a vibration signal componentdependent on a vibrational movement of a rotationally movable part inthe gear system 700. The sensor 10 is delivers the analogue signalS_(EA) to a signal processing arrangement 920.

Signal processing arrangement 920 may include a sensor interface 40 anda data processing means 50. The sensor interface 40 includes an A/Dconverter 44 (FIG. 2A, FIG. 2B) generating the digital measurementsignal S_(MD). The A/D converter 44 is coupled to the data processingmeans 50 so as to deliver the digital measurement data signal S_(MD) tothe data processing means 50.

The data processing means 50 is coupled to a user interface 102. Theuser interface 102 may include user input means 104 enabling a user toprovide user input. Such user input may include selection of a desiredanalysis function 105, 290, 290T, 290F (FIG. 4, FIG. 7, FIG. 8), and/orsettings for signal processing functions 94, 250, 310, 470, 470A, 470,B,320, 294 (See FIG. 4, FIG. 30).

The user interface 102 may also include a display unit 106, as describede.g. in connection with FIG. 2A an FIG. 5.

FIG. 30 is a block diagram illustrating the parts of the signalprocessing arrangement 920 of FIG. 29 together with the user interface102, 104 and the display 106.

The sensor interface 40 comprises an input 42 for receiving an analoguesignal S_(EA) from a Shock Pulse Measurement Sensor and an A/D converter44. A signal conditioner 43 (FIG. 2B) may optionally also be provided.The A/D converter 44 samples the received analogue signal with a certainsampling frequency f_(S) so as to deliver a digital measurement datasignal S_(MD) having said certain sampling frequency f_(S).

The sampling frequency f_(S) may be set tof _(S) =k*f _(SEAmax)wherein

-   -   k is a factor having a value higher than 2.0

Accordingly the factor k may be selected to a value higher than 2.0.Preferably factor k may be selected to a value between 2.0 and 2.9 inorder to avoid aliasing effects. Selecting factor k to a value higherthan 2.2 provides a safety margin in respect of aliasing effects, asmentioned above in this document. Factor k may be selected to a valuebetween 2.2 and 2.9 so as to provide said safety margin while avoidingto generate unnecessarily many sample values. According to an embodimentthe factor k is advantageously selected such that 100*k/2 renders aninteger. According to an embodiment the factor k may be set to 2.56.Selecting k to 2.56 renders 100*k=256=2 raised to 8.

According to an embodiment the sampling frequency f_(S) of the digitalmeasurement data signal S_(MD) may be fixed to a certain value f_(S),such as e.g. f_(S)=102.4 kHz

Hence, when the sampling frequency f_(S) is fixed to a certain valuef_(S), the frequency f_(SEAmax) of the analogue signal S_(EA) will be:f _(SEAmax) =f _(S) /kwherein f_(SEAmax) is the highest frequency to be analyzed in thesampled signal.

Hence, when the sampling frequency f_(S) is fixed to a certain valuef_(S)=102.4 kHz, and the factor k is set to 2.56, the maximum frequencyf_(SEAmax) of the analogue signal S_(EA) will be:f _(SEAmax) =f _(S) /k=102 400/2.56=40 kHz

The digital measurement data signal S_(MD) having sampling frequencyf_(S) is received by a filter 240. According to an embodiment, thefilter 240 is a high pass filter having a cut-off frequency f_(LC). Thisembodiment simplifies the design by replacing the band-pass filter,described in connection with FIG. 6, with a high-pass filter 240. Thecut-off frequency f_(LC) of the high pass filter 240 is selected toapproximately the value of the lowest expected mechanical resonancefrequency value f_(RMU) of the resonant Shock Pulse Measurement sensor10. When the mechanical resonance frequency f_(RM) is somewhere in therange from 30 kHz to 35 kHz, the high pass filter 240 may be designed tohaving a lower cutoff frequency f_(LC)=30 kHz. The high-pass filteredsignal is then passed to the rectifier 270 and on to the low pass filter280.

According to an embodiment it should be possible to use sensors 10having a resonance frequency somewhere in the range from 20 kHz to 35kHz. In order to achieve this, the high pass filter 240 may be designedto having a lower cutoff frequency f_(LC)=20 kHz.

The output signal from the digital filter 240 is delivered to a digitalenveloper 250.

Whereas prior art analogue devices for generating an envelop signal inresponse to a measurement signal employs an analogue rectifier whichinherently leads to a biasing error being introduced in the resultingsignal, the digital enveloper 250 will advantageously produce a truerectification without any biasing errors. Accordingly, the digitalenvelop signal S_(ENV) will have a good Signal-to-Noise Ratio, since thesensor being mechanically resonant at the resonance frequency in thepassband of the digital filter 240 leads to a high signal amplitude.Moreover, the signal processing being performed in the digital domaineliminates addition of noise and eliminates addition of biasing errors.

According an embodiment of the invention the optional low pass filter280 in enveloper 250 may be eliminated. In effect, the optional low passfilter 280 in enveloper 250 is eliminated since decimator 310 includes alow pass filter function. Hence, the enveloper 250 of FIG. 30effectively comprises a digital rectifier 270, and the signal producedby the digital rectifier 270 is delivered to integer decimator 310,which includes low pass filtering.

The Integer decimator 310 is adapted to perform a decimation of thedigitally enveloped signal S_(ENV) so as to deliver a digital signalS_(RED) having a reduced sample rate f_(SR1) such that the output samplerate is reduced by an integer factor M as compared to the input samplerate f_(S).

The value M may be settable in dependence on a detected speed ofrotation f_(ROT). The decimator 310 may be settable to make a selecteddecimation M:1, wherein M is a positive integer. The value M may bereceived on a port 404 of decimator 310.

The integer decimation is advantageously performed in plural steps usingLow pass Finite Impulse Response Filters, wherein each FIR Filter issettable to a desired degree of decimation. An advantage associated withperforming the decimation in plural filters is that only the last filterwill need to have a steep slope. A steep slope FIR filter inherentlymust have many taps, i.e. a steep FIR filter must be a long filter. Thenumber of FIR taps, is an indication of

-   -   1) the amount of memory required to implement the filter,    -   2) the number of calculations required, and    -   3) the amount of “filtering” the filter can do; in effect, more        taps means more stopband attenuation, less ripple, narrower        filters, etc. Hence the shorter the filter the faster it can be        executed by the DSP 50. The length of a FIR filter is also        proportional to the degree of achievable decimation. Therefore,        according to an embodiment of the integer decimator, the        decimation is performed in more than two steps.

According to a preferred embodiment the integer decimation is performedin four steps: M1, M2, M3 & M4. The total decimation M equalsM1*M2*M3*M4 This may achieved by providing a bank of different FIRfilters, which may be combined in several combinations to achieve adesired total decimation M. According to an embodiment there are eightdifferent FIR filters in the bank.

Advantageously, the maximum degree of decimation in the last, 4:th, stepis five (M4=5), rendering a reasonably short filter having just 201taps. In this manner the FIR filters in steps 1, 2 and 3 can be allowedto have an even lower number of taps. In fact this allows for thefilters in steps 1, 2 and 3 to have 71 taps each or less. In order toachieve a total decimation of M=4000, it is possible to select the threeFIR-filters providing decimation M1=10, M2=10 and M3=10, and the FIRfilter providing decimation M4=4. This renders an output sample ratef_(SR1)=25.6, when f_(S)=102400 Hz, and a frequency range of 10 Hz.These four FIR filters will have a total of 414 taps, and yet theresulting stopband attenuation is very good. In fact, if the decimationof M=4000 were to be made in just one single step it would have requiredabout 160 000 taps to achieve an equally good stop band attenuation.

Output 312 of integer Decimator 310 is coupled to fractional decimator470 and to an input of a selector 460. The selector enables a selectionof the signal to be input to the enhancer 320.

When condition monitoring is made on a rotating part having a constantspeed of rotation, the selector 460 may be set in the position todeliver the signal S_(RED) having sample frequency f_(SR1) to the input315 of enhancer 320, and fractional decimator 470 may be disabled. Whencondition monitoring is made on a rotating part having a variable speedof rotation, the fractional decimator 470 may be enabled and theselector 460 is set in the position to deliver the signal S_(RED2)having sample frequency f_(SR2) to the input 315 of enhancer 320.

The fractional decimator 470 may be embodied by fractional decimator470B, 94 including an adaptable FIR filter 608, as described inconnection with FIGS. 20, 21 and 22 and FIG. 4.

The fractional decimator 470 is coupled to deliver a decimated signalS_(RED2) having the lower sample rate f_(SR2) to the selector 460, sothat when the condition analyzer is set to monitor a machine withvariable speed of rotation, the output from fractional decimator 470B isdelivered to enhancer 320.

Enhancer 320, 94 may be embodied as described in connection with FIGS.10A, 10B, 11, 12 and 13 and FIG. 4. The measuring signal input to theenhancer 320 is the signal S_(RED) (See FIG. 30), which is alsoillustrated in FIG. 11 as having I_(LENGTH) sample values. The signalS_(RED) is also referred to as I and 2060 in the description of FIG. 11.The Enhancer signal processing involves discrete autocorrelation for thediscrete input signal S_(RED). The output signal O, also referred to asS_(MDP) is illustrated in FIGS. 12 and 13.

The measurement signal S_(RED1), S_(RED), to be input to the enhancer,may include at least one vibration signal component S_(D) dependent on avibration movement of said rotationally movable part; wherein saidvibration signal component has a repetition frequency f_(D) whichdepends on the speed of rotation f_(ROT) of said first part. Therepetition frequency f_(D) of signal component S_(D) may be proportionalto the speed of rotation f_(ROT) of the monitored rotating part.

Two different damage signatures SD1, SD2 may have different frequenciesfd1, fd2 and still be enhanced, i.e. SNR-improved, by the enhancer.Hence, the enhancer 320 is advantageously adapted to enhance differentsignatures S_(D1), S_(D2) having mutually different repetitionfrequencies f_(D1) and f_(D2). Both of the repetition frequencies f_(D1)and f_(D2) are proportional to the speed of rotation f_(ROT) of themonitored rotating part, while f_(D1) is different from f_(D2) (f_(D1)<>f_(D2)). This may be expressed mathematically in the following manner:fD1=k1*f _(ROT), andfD2=k2*f _(ROT), wherein

-   -   k1 and k2 are positive real values, and    -   k1< >k2, and    -   k1 greater than or equal to one (1), and    -   k2 greater than or equal to one (1)

The enhancer delivers an output signal sequence to an input of timedomain analyzer 290T, so that when a user selects, via user interface102,104 to perform a time domain analysis, the time domain analyzer290T, 105 (FIG. 30 & FIG. 4) will execute the selected function 105 anddeliver relevant data to the display 106. An advantage with the enhancer320 is that it delivers the output signal in the time domain. Hence,condition monitoring functions 105, 290T requiring an input signal inthe time domain can be set to operate directly on the signal values ofthe signal output illustrated in FIGS. 12 and 13.

When a user selects, via user interface 102,104 to perform a frequencydomain analysis, the enhancer will deliver the output signal sequence toFast Fourier Transformer 294, and the FFTransformer will deliver theresulting frequency domain data to the frequency domain analyzer 290F,105 (FIG. 30 & FIG. 4). The frequency domain analyzer 290F, 105 willexecute the selected function 105 and deliver relevant data to thedisplay 106.

In the embodiment shown in FIGS. 29 and 30, it is advantageously easyfor a user to do perform an analysis employing the enhancer and thefractional decimator.

The Below is an Example of Parameter Settings:

In order to perform an analysis in the frequency domain the user mayinput the following data via user interface 102,104:

1) Information indicative of the highest repetition frequency f_(D) ofinterest. The repetition frequency f_(D) is repetition frequency asignature SD of interest. This information may be input in the form of afrequency or in the form an order number O_(vHigh) indicative of thehighest repetition frequency of damage signature SD of interest.2) Information indicative of the desired improvement of the SNR valuefor repetitive signal signature S_(D). This information may be input inthe form the SNR Improver value L. The SNR Improver value L is alsodiscussed below, and in connection with FIG. 10A above.3) Information indicative of the desired frequency resolution in the FFT294, when it is desired to perform an FFT of the signal output fromenhancer. This may be set as value Z frequency bins. According to anembodiment of the invention, the frequency resolution Z is settable byselecting one value Z from a group of values. The group of selectablevalues for the frequency resolution Z may includeZ=400Z=800Z=1600Z=3200Z=6400

Hence, although the signal processing is quite complex, the arrangement920 has been designed to provide an advantageously simple user interfacein terms of information required by the user. When the user inputs orselects values for the above three parameters, all the other values areautomatically set or preset in the arrangement 920.

The SNR Improver Value L

The signal to be input to the enhancer may include a vibration signalcomponent dependent on a vibration movement of the rotationally movablepart; wherein said vibration signal component has a repetition frequencyf_(D) which depends on the speed of rotation f_(ROT) of said first part;said measurement signal including noise as well as said vibration signalcomponent so that said measurement signal has a first signal-to-noiseratio in respect of said vibration signal component. The enhancerproduces an output signal sequence (O) having repetitive signalcomponents corresponding to said at least one vibration signal componentso that said output signal sequence (O) has a second signal-to-noiseratio value in respect of said vibration signal component. The inventorhas established by measurements that the second signal-to-noise ratiovalue is significantly higher than the first signal-to-noise ratio whenthe SNR Improver value L is set to value one (1).

Moreover, the inventor has established by measurements that when the SNRImprover value L is increased to L=4, then the resulting SNR value inrespect of said vibration signal component in the output signal isdoubled as compared to the SNR value associated with L=1. Increasing theSNR Improver value L to L=10 appears to render an improvement of theassociated SNR value by a factor 3 for the vibration signal component inthe output signal, as compared to the SNR value for same input signalwhen L=1. Hence, when increasing SNR Improver value L from L₁=1 to L₂the resulting SNR value may increase by the square root of L₂.

Additionally the user may input a setting to have the arrangement 920keep repeating the measurement. The user may set it to repeat themeasurement with a certain repetition period T_(PM), i.e. to alwaysstart a new measurement when the time T_(PM) has passed. T_(PM) may beset to be a one week, or one hour or ten minutes. The value to selectfor this repetition frequency depends on the relevant measuringconditions.

Since the enhancer method requires a lot of data input values, i.e. thenumber of input sample values may be high, and it is suited formeasuring on slowly rotating parts, the duration of the measurement willsometimes be quite long. Hence there is a risk that the user settingsfor the frequency of repetition of measurements is incompatible with theduration of measurements. Therefore, one of the steps performed by thearrangement 920, immediately after receiving the above user input, is tocalculate an estimate of the expected duration of measurements T_(M).

The duration T_(M) is:T _(M) =I _(LENGTH) /f _(SR2),

-   -   Wherein I_(LENGTH) is the number of samples in the signal to be        input into the enhancer in order to achieve measurements        according to selected user settings as defined below, and fSR2        is as defined below.

The arrangement 920 is also adapted to compare the duration ofmeasurements T_(M) with the repetition period value T_(PM) as selectedby the user. If the repetition period value T_(PM) is shorter or aboutthe same as the expected duration of measurements T_(M), a parametercontroller 930 is adapted to provide a warning indication via the userinterface 102,106 e.g. by a suitable text on the display. The warningmay also include a sound, or a blinking light.

According to an embodiment the arrangement 920 is adapted to calculate asuggested minimum value for the repetition period value T_(PM) isdependence on the calculated estimate of duration of measurements T_(M).

Based on the above user settings, the parameter controller 930 of signalprocessing arrangement 920 is capable of setting all the parameters forthe signal processing functions 94 (FIG. 4), i.e. integer decimatorsettings and enhancer settings. Moreover the parameter controller 930 iscapable of setting all the parameters for the fractional decimator whenneeded. The parameter controller 930 is capable of setting the parameterfor the FFT 294 when a frequency analysis is desired.

The Following Parameter May be Preset in the Arrangement 920 (FIG. 30):

sample frequency f_(S) of A/D converter 40,44.

The Following Parameter May be Measured: f_(ROT)

As mentioned above, the parameter value f_(ROT) may be measured andstored in association with the corresponding sample values of the signalS_(RED1) whose sample values are fed into the fractional decimator 470B.

The Following Parameters May be Automatically Set in the Arrangement920:

Sample rate in the signal output from enhancer 320:f _(SR2) =C*Ov*f _(ROT)

-   -   wherein        -   C is a constant of value higher than 2.0        -   O_(v) is the order number input by the user, or calculated            in response to a highest frequency value to be monitored as            selected by the user        -   f_(ROT) is the momentary measured rotational speed of the            rotating part during the actual condition monitoring;

M=The integer decimator value for use in decimator 310 is selected froma table including a set of predetermined values for the total integerdecimation. In order to select the most suitable value M, the parametercontroller 930 (FIG. 30) first calculates a fairly close valueM_calc=f _(S) /f _(SR2) *f _(ROTmin) /f _(ROTmax)

-   -   wherein        -   f_(S) & f_(SR2) are defined above, and        -   f_(ROTmin)/f_(ROTmax) is a value indicative of the relation            between lowest and highest speed of rotation to be allowed            during the measurement.

Based on the value M_calc the selector then chooses a suitable value Mfrom a list of preset values. This may e.g be done by selecting theclosest value M which is lower than M_calc from the table mentionedabove.

f_(SR1)=the sample rate to be delivered from the integer decimator 310.fSR1 is set to f_(SR1)=fS/M

D is the fractional decimator value for fractional decimator. D may beset to D=fsr1/fsr2, wherein fsr1 and fsr2 are as defined above.O _(LENGTH) =C*Z

-   -   wherein        -   C is a constant of value higher than 2.0, such as e.g. 2.56            as mentioned above        -   Z is the selected number of frequency bins, i.e. information            indicative of the desired frequency resolution in the FFT            294, when it is desired to perform an FFT of the signal            output from enhancer.

S_(START)=O_(LENGTH) or a value higher than O_(LENGTH), whereinO_(LENGTH) is as defined immediately above.I _(Length) =O _(LENGTH) *L+S _(START) +O _(LENGTH)C _(Length) =I _(LENGTH) −S _(START) −O _(LENGTH)

SMDP(t)=the values of the samples of the output signal, as defined inequation (5) (See FIG. 10A).

Hence, the parameter controller 930 is adapted to generate thecorresponding setting values as defined above, and to deliver them tothe relevant signal processing functions 94 (FIG. 30 & FIG. 4).

Once an output signal has been generated by enhancer 320, the conditionanalyser 290 can be controlled to perform a selected condition analysisfunction 105, 290, 290T, 290F by means of a selection signal deliveredon a control input 300 (FIG. 30). The selection signal delivered oncontrol input 300 may be generated by means of user interaction with theuser interface 102 (See FIGS. 2A & 30). When the selected analysisfunction includes Fast Fourier Transform, the analyzer 290F will be setby the selection signal 300 to operate on an input signal in thefrequency domain.

The FFTransformer 294 may be adapted to perform Fast Fourier Transformon a received input signal having a certain number of sample values. Itis advantageous when the certain number of sample values is set to aneven integer which may be divided by two (2) without rendering afractional number.

According to an advantageous embodiment of the invention, the number ofsamples O_(LENGTH) in the output signal from the enhancer is set independence on the frequency resolution Z. The relation between frequencyresolution Z and the number of samples O_(LENGTH) in the output signalfrom the enhancer is:O _(LENGTH) =k*Z

-   -   wherein        -   O_(LENGTH) is the samples number of sample values in the            signal delivered from the enhancer 320.        -   k is a factor having a value higher than 2.0

Preferably factor k may be selected to a value between 2.0 and 2.9 inorder to provide a good safety margin while avoiding to generateunnecessarily many sample values. According to an embodiment the factork is advantageously selected such that 100*k/2 renders an integer. Thisselection renders values for O_(LENGTH) that are adapted to be suitableas input into the FFTransformer 294. According to an embodiment thefactor k may be set to 2.56. Selecting k to 2.56 renders 100*k=256=2raised to 8.

Table A indicates examples of user selectable Frequency resolutionvalues Z and corresponding values for O_(LENGTH).

TABLE A k Z O_(LENGTH) 2.56 400 1024 2.56 800 2048 2.56 1600 4096 2.563200 8192 2.56 6400 16384 2.56 12800 32768 2.56 25600 65536 2.56 51200131072

The invention claimed is:
 1. An apparatus for detecting a condition of abearing associated with a shaft of a machine, where the shaft rotates ata variable rotational speed during operation of the machine, and wherethe bearing includes a rolling element placed between an outer racebearing surface and an inner race bearing surface so that, when theshaft rotates, the rolling element movable between said outer and innerrace bearing surfaces thereby causing mechanical vibration, theapparatus comprising: (A) a vibration sensor configured to generate ananalogue vibration signal including a vibration signal componentdependent on said mechanical vibration, and stochastic signalcomponents, where the vibration signal component has a repetitionfrequency that depends on the variable rotational speed of the shaft;(B) an analogue-to-digital converter that samples said analoguevibration signal at a first sampling frequency so as to generate adigital vibration signal based on said analogue vibration signal; (C) adigital enveloper that generates a digital envelope signal based on saiddigital vibration signal, the digital envelope signal including a signalsignature that is periodic with a frequency that depends on saidrepetition frequency; (D) a decimator that generates a decimated digitalenvelope signal having a reduced sampling frequency, where saiddecimator has a first input for receiving said digital envelope signal;a second input for receiving a signal indicative of the variablerotational speed of the shaft; and a third input for receiving a signalindicative of an output sample rate setting signal; said decimator beingadapted to generate said decimated digital envelope signal dependent onsaid digital envelope signal, said signal indicative of the variablerotational speed of the shaft, and said signal indicative of an outputsample rate setting signal, such that the number of sample values perrevolution of the shaft is kept at a substantially constant value whenthe variable rotational speed of the shaft varies during a measurement,thereby generating said decimated digital envelope signal so that saidperiodic signal signature has a periodicity in terms of repetition perrevolution of the shaft; (E) an enhancer having an input for receivingsaid decimated digital envelope signal, said enhancer being adapted toperform signal processing involving autocorrelation of said decimateddigital envelope signal so as to produce an autocorrelated envelopesignal sequence where said periodic signal signature is amplified inrelation to stochastic signal components; and (F) an evaluator thatdetects said condition of the bearing based on said autocorrelatedenvelope signal sequence, said evaluator thereby detecting said bearingcondition even when the variable rotational speed of the shaft variesduring a measurement.
 2. The apparatus according to claim 1, whereinsaid evaluator delivers an output signal indicative of at least onebearing condition selected from a group comprising: an inner racebearing surface damage, and an outer race bearing surface damage.
 3. Theapparatus according to claim 1, wherein said vibration sensor is amechanically resonant vibration sensor.
 4. The apparatus according toclaim 1, wherein said enhancer interacts with said decimator to producesaid autocorrelated envelope signal sequence such that said evaluatordetects said bearing condition when the variable rotational speed of theshaft is lower than 50 revolutions per minute.
 5. The apparatusaccording to claim 1, wherein said decimated digital envelope signal hasa first signal-to-noise ratio value in respect of said periodic signalsignature, wherein said autocorrelated envelope signal sequence has asecond signal-to-noise ratio value in respect of said periodic signalsignature, and wherein said enhancer is adapted to produce saidautocorrelated envelope signal sequence so that said secondsignal-to-noise ratio value is higher than said first signal-to-noiseratio value such that said evaluator detects said bearing condition whenthe variable rotational speed of the shaft is lower than 50 revolutionsper minute.
 6. The apparatus according to claim 1, further comprising: auser interface for setting a signal-to-noise ratio improver value,wherein said decimated digital envelope signal has a firstsignal-to-noise ratio value in respect of said periodic signalsignature, wherein said autocorrelated envelope signal sequence has asecond signal-to-noise ratio value in respect of said periodic signalsignature, wherein said enhancer is adapted to produce saidautocorrelated envelope signal sequence so that said secondsignal-to-noise ratio value is higher than said first signal-to-noiseratio value, and wherein said enhancer is adapted to produce saidautocorrelated envelope signal sequence so that said secondsignal-to-noise ratio depends on said signal-to-noise ratio improvervalue.
 7. The apparatus according to claim 1, wherein said evaluatordelivers an output signal indicative of whether said bearing conditionis any of normal, deteriorated, and abnormal.
 8. The apparatus accordingto claim 1, wherein said enhancer is configured to deliver saidautocorrelated envelope signal sequence as a time domain signal.
 9. Theapparatus according to claim 1, wherein said enhancer is configured toachieve said correlation by a method for producing autocorrelation databy performing successive Fourier Transform operations on said decimateddigital envelope signal to provide said autocorrelated envelope signal.10. The apparatus according to claim 1, wherein said enhancer is adaptedto receive a first plurality of samples of said decimated digitalenvelope signal, said decimated digital envelope signal representingmechanical vibrations emanating from rotating the shaft so far as tocause occurrence of a vibration having a period of repetition, whereinsaid enhancer is adapted to divide said decimated digital envelopesignal into a first signal portion and a second signal portion, andwherein said enhancer is adapted to generate said autocorrelatedenvelope signal sequence so that it has a second plurality of samples,said second plurality being a positive integer and lower than said firstplurality, said autocorrelated envelope signal sequence being generatedbased on the first signal portion and the second signal portion.
 11. Theapparatus according to claim 1, wherein said decimator comprises a firstdecimator for performing a first decimation of the digital envelopesignal so as to achieve a first digital signal having a first reducedsampling frequency, the first reduced sampling frequency being reducedby an integer factor as compared to the first sampling frequency, and asecond decimator for performing a second decimation of said firstdigital signal so as to generate said decimated digital envelope signal.12. The apparatus according to claim 11, wherein said integer factor issettable to a value adapted for measuring conditions in a measuringsession, said integer factor being kept constant for the duration of ameasuring session.
 13. The apparatus according to claim 1, wherein saidperiodicity of the periodic signal signature is expressed as a factorvalue, said factor value being a positive, real number not limited tointeger numbers.
 14. The apparatus according to claim 13, wherein saidfactor value reflects the number of repetitions of said periodic signalsignature per revolution of the shaft as a real number not limited tointeger numbers.
 15. The apparatus according to claim 1, wherein saidvibration sensor has a substantially linear response in a frequencyrange up to 10 kHz.
 16. A method for detecting a condition of a bearingassociated with a shaft of a machine, the method comprising the stepsof: (I) providing the apparatus according to claim 1; (II) applying thevibration sensor to a measuring point on the machine; and (III)generating the bearing condition from the evaluator, wherein variablerotational speed of the shaft is less than 50 revolutions per minute.17. An apparatus for detecting a condition of a gear system associatedwith a shaft of a machine, where the shaft rotates at a variablerotational speed during operation of the machine and the gear systemincludes a first gear having first teeth and a second gear having secondteeth so that, when the shaft rotates, the first teeth engage with thesecond teeth thereby causing mechanical vibration, the apparatuscomprising: (A) a vibration sensor configured to generate an analoguevibration signal including a vibration signal component dependent onsaid mechanical vibration, and stochastic signal components, where thevibration signal component has a repetition frequency that depends onthe variable rotational speed of the shaft; (B) an analogue-to-digitalconverter for sampling said analogue vibration signal at a firstsampling frequency so as to generate a digital vibration signal based onsaid analogue vibration signal; (C) a digital enveloper for generating adigital envelope signal based on said digital vibration signal, thedigital envelope signal including a signal signature that is periodicwith a frequency that depends on said repetition frequency; (D) adecimator for generating a decimated digital envelope signal having areduced sampling frequency, where said decimator has a first input forreceiving said digital envelope signal, a second input for receiving asignal indicative of the variable rotational speed of the shaft, and athird input for receiving a signal indicative of an output sample ratesetting signal, said decimator being adapted to generate said decimateddigital envelope signal dependent on said digital envelope signal, saidsignal indicative of the variable rotational speed of the shaft, andsaid signal indicative of an output sample rate setting signal, suchthat the number of sample values per revolution of the shaft is kept ata substantially constant value when the variable rotational speed of theshaft varies during a measurement, thereby generating said decimateddigital envelope signal so that said periodic signal signature has aperiodicity in terms of repetition per revolution of the shaft; (E) anenhancer having an input for receiving said decimated digital envelopesignal; said enhancer being adapted to perform signal processinginvolving autocorrelation of said decimated digital envelope signal soas to produce an autocorrelated envelope signal sequence wherein saidperiodic signal signature is amplified in relation to stochastic signalcomponents; and (F) an evaluator for detecting said gear systemcondition based on said autocorrelated envelope signal sequence, saidevaluator thereby detecting said gear system condition even when thevariable rotational speed of the shaft varies during a measurement. 18.The apparatus according to claim 17, wherein said evaluator delivers anoutput signal indicative of a teeth engagement error.
 19. The apparatusaccording to claim 17, wherein said periodicity of the periodic signalsignature is expressed as a factor value, said factor value being apositive, real number not limited to integers.
 20. The apparatusaccording to claim 19, wherein said factor value reflects the number ofrepetitions of said periodic signal signature per revolution of theshaft as a real number not limited to integer numbers.
 21. The apparatusaccording to claim 17, wherein the gear system further comprises abearing associated with said shaft, said bearing including a rollingelement placed between an outer race bearing surface and an inner racebearing surface so that, when the shaft rotates, the rolling elementmoves between said outer and inner race bearing surfaces, therebycausing mechanical vibration, and wherein said evaluator, based on saidperiodicity, delivers an output signal indicative of at least onedeteriorated condition selected from a group comprising: a Bearing InnerRace damage, a Bearing Outer Race damage, and a teeth engagement error.22. The apparatus according to claim 17, wherein said enhancer interactswith said decimator to produce said autocorrelated envelope signalsequence such that said evaluator detects said gear system conditioneven when the variable rotational speed of the shaft is lower than 50revolutions per minute.